The 6-year-old boy with a cap of soft brown hair and a grin that could melt Stonehenge had climbed to the top of a 30-foot-high playground slide, but he wasn't quite ready to take the plunge. Something was missing; he didn't have an audience. ''Papa, Papa,'' he called down excitedly. ''Watch me, watch me.'' Below, his doting father, wearing a gray herringbone coat, looked up proudly and smiled. ''Go on, then. I'm watching.''

Prince Charles isn't the only one watching the little boy at the top of the slide. William Arthur Phillip Louis, H.R.H. Prince William of Wales -- the tyke, who according to laws of succession, will one day become King William V -- turns 7 this Wednesday, June 21, and the whole world will be looking on. Up to now, he has been raised by Charles and Diana in as normal a manner as possible. But as Willie the Wombat -- his dad's moniker for him -- approaches his 7th year, the traditional age of reason, he's beginning to understand that he is anything but a normal child. Normal children, after all, don't have cameras trained on them at every turn. Normal children don't have articles written about them. Normal children don't get a department store like Harrods all to themselves to do their Christmas shopping.

Once known as the Prince of Wails (particularly when he didn't get his way), Wills, as his mother calls him, is slowly shedding his willful image and turning into a little charmer, gradually adapting to the knowledge that he has a big future in store. ''William knows he's special,'' says a recent visitor to the family's country estate at Highgrove in Gloucestershire. ''He's incredibly confident for a little boy his age. It's a bit like talking to another adult.''

Adult was not the word that would have sprung to mind when describing the old William, known by the British press as the Basher. Recent generations of royal progeny have been ready for formal appearances by age 3. Not so Wills, who even at 4 and 5 was rarely trusted to behave in public. His first major gig -- page boy at his Uncle Andy's 1986 wedding to Sarah Ferguson -- didn't exactly provide evidence that here was a kid with the regal stuff. He fidgeted, squirmed and whispered all through the ceremony. This was hardly a child poised to wear the purple.

Today, happily, Wills is a changed child. Some in the family retinue credit his transformation to the replacement of indulgent nanny Barbara Barnes with sterner Ruth Wallace (called Roof by brother Harry, 4); others say he has simply matured. All agree that the days of Wills's king-size tantrums appear to be over. And if he does sometimes type ''naughty'' words like wee and bottom (which he spelled botem) onto the computer screen at his exclusive pre- prep school, Wetherby, what 6-year-old, royal or otherwise, is incapable of that?

''William is being molded into a delightful young person,'' says a Windsor family friend. ''Diana supplies the comfort and security of a solid, loving family existence. Charles supplies the discipline he will need through his whole life.''

* Of course, Charles isn't always around, given his love of fishing in Scotland and painting in Italy -- and the royal tradition of distant parenting. According to one longtime observer of the royal family, ''The children get upset when Charles goes away for so long, but I honestly don't think William has suffered too much because of these absences. Charles does blow hot and cold in his relationship with both children, but when they are all at Highgrove, full family life is experienced.'' At Highgrove, Charles takes his sons fishing, chaperones them on tractor rides and walks them around the fields, sharing his love of nature and his environmental concerns.

Clearly, Mom, Dad and Nanny are doing something right, since William is already showing qualities of leadership. He's even evincing an instinctive flair for one of its perks: ordering people around. ''Prince William can be a really bossy-boots,'' says the mother of one of his classmates. ''I suppose it shouldn't be surprising. But he is a natural leader and likes to take command. He likes to organize games of tag that can get quite boisterous.'' A few months ago, William served as a page boy at the wedding of Diana's cousin Edward Barry. Perhaps because he considered himself an old hand at the job -- remember Fergie's wedding -- he began barking commands to the other five page boys, among them Harry, and the four tiny, utterly awestruck bridesmaids. ''Get back! Get in line!'' William shouted to his short subjects as they all waited outside the church.

As Harry has apparently learned, having a brother who's a monarch-in- training is almost like having a third parent. Last summer it was Wills, not Diana, who spotted Harry sticking his tongue out at the press after a visit to see newborn cousin Beatrice at Portland Hospital. Immediately he grabbed the royal rascal and hissed, ''Stop it, Harry. That's very naughty.'' Yet, lest anyone think King Tot is turning into a tyrant, William has also absorbed certain crucial tenets of royal deportment, like putting yourself in the other guy's shoes. ''I've often seen him comforting a young child who's clearly unhappy,'' says the schoolmate's mom. ''He'll talk earnestly to him and make sure he's all right before resuming playing. He really does think of others.''

He's no less protective of Harry. That was clear the day Charles took the boys to the Windsor Safari Park, near the castle, to play on the giant slide. ''William was absolutely charming,'' said a woman who was waiting in line with her own children. ''He very much took over as leader and was really encouraging. He said to his brother, 'Harry, Harry, take your coat off. You will go much quicker.' He looked after him and was very gentle.''

While learning the rudiments of statesmanship, William is acquiring another kingly skill: horsemanship. He competed in his first horse show a year ago, won a rosette as the third-best young rider at his level and has recently won another rosette for best-turned-out rider. He saddles up nearly every weekend at Highgrove on his favorite pony, Trigger. And at his weekly teas with the Queen, the chatter almost always centers on ponies.

Her Majesty can attest to his equestrian skills. Last fall, Charles and Wills had just wrapped up a riding lesson when they trotted past Elizabeth leaving the Balmoral Castle stables on her favorite horse, a bay called Greenshield. ''Where are you going, Granny?'' called William, mounted on Trigger. ''Can I come with you?''

According to an observer, Elizabeth turned and beckoned to her grandson. Later at a picnic lunch, she laughed as she told everyone, ''William trotted along so fast on his pony I could barely keep up. I thought the bay would pitch me head first into the road at any minute.''

''Few people can lead the Queen on a merry chase,'' noted this source. ''But William is one. He has inherited his mother's beguiling manner along with the Spencer looks, and like most of the women in his life, his grandmother simply adores him.''

Wills's academic performance, on the other hand, is not quite on a par with his horsemanship. But his presence in Wetherby's less-advanced group, the class known as form Three Red, causes no distress to his parents. ''Being too bright can be a positive disadvantage for the sort of life that William has before him,'' Charles has said. ''We're open-minded about William and his education. I would like to try and bring up our children to be well mannered, to think of other people, to put themselves in other people's positions. That way, even if they turn out not to be very bright or very qualified, at least if they have reasonable manners, they will get so much further in life than if they did not have any at all.''

At least Wetherby has taught its most celebrated pupil one thing -- how to write a creditable letter. When his folks went off to the Arab Emirates this spring, there was a surprise message waiting for them in a suitcase: a carefully penned note from their No. 1 son. ''Dear Mummy and Papa,'' it read. ''I hope you have a lovely time on your tour. But come home soon. I miss you. Lots of love, William.'' Two kisses adorned the bottom of the epistle. ''The princess was so touched by her son's letter,'' said Sheikha Mariem, one of Diana's hostesses on the state visit. ''She said it was the first one he had ever written to his parents.''

Many more may follow. By September 1990, Diana and Charles will have to decide the nature of William's education beyond Wetherby. Charles and the Queen are said to believe William is ready to go off to boarding school, while Diana thinks he's still too young. There is speculation that Wills will soon enroll at St. Paul's, a day school in West London, before moving on at 13 to Eton, where Diana's father and brother were educated. It seems certain that William will not go to his father and grandfather's alma mater, Gordonstoun in Scotland, which Charles considers too spartan and remote.

William himself might eventually like to follow in the footsteps of Uncle Andy, who went to the Royal Naval College at Dartmouth. From the time he was a toddler, a royal aide recalls, Wills watched wide-eyed as army officers saluted Prince Charles, and he began copying their salute. ''He still makes Harry line up with him at the front door, and they snap to attention when their father goes out,'' says the aide. ''The Prince of Wales is tickled pink by their antics, but he always returns the salute with a straight face.''

Prince William's fondness for martial matters is evident. A few years ago he received a uniform like the one worn in the Parachute Regiment, of which his father is colonel-in-chief. Since then, says an observer, ''He has saluted anyone he encountered in uniform.'' And while other boys at Wetherby content themselves with painting beach scenes and futuristic cars, Wills sketches castles with yellow splotches in the battlements, which, as he points out to his mates, represent gunfire.

Last summer the Gordon Highlanders, on guard at Scotland's Balmoral Castle, realized they were dealing with a kindred spirit and, as a special treat, made William a member of the regiment for a day. This meant suiting up in specially tailored camouflage gear, learning to handle a gun and dining at a field kitchen up in the hills. ''I really love soldiers' food,'' William told an envious Harry on his return to civilian life.

) Such is the life -- by turns both extraordinarily glorious and typically boyish -- of the future King as he approaches his birthday. He's now going through a phase few people experience, a period of recognizing that he has been set apart by destiny. ''I can't pinpoint any particular moment,'' Charles said of his own realization. ''I didn't suddenly wake up in my pram and say, 'Yippee.' I think it just dawns on you slowly, that people are interested, and slowly you get the idea that you have a certain duty and responsibility.''

And so it will be a different Wills who sits down to his birthday cake this year -- more the budding little King than the bad little boy. The grand celebration will be held at his London home in Kensington Palace. Jell-O (which the British persist in calling jelly), strawberries and cakes will be served. The select group in attendance will include a dozen of the prince's buddies from Wetherby, the children of Diana's two sisters and perhaps Aunt Anne's children, Peter and Zara Phillips. If William has his way -- and he probably will, it's his birthday -- there will be a few rowdy rounds of tag. Later, following the age-old custom, the prince will blow out the candles on his cake. Some of those gathered around will no doubt wonder what he could possibly be wishing for, but that's easy: Like most 7-year-olds, he will ask for the world. The wonder and the challenge of being William is that his wishes may well come true.

People Weekly- June 26, 1989