Some of my favourites (they don't all have websites, we like the smaller ones with fewer facilities):
Wasdale head in the lakes. streams for paddling and damming, trees, mountains. cars kept off site. idyllic when weather good.
Coniston Hall campsite, Coniston. a bit busy but right on the beach, great for playing or watersports.
Red Squirrel in Glencoe. if the mists clear.
Patterdale, on Ullswater. again, right on the lakeside, little beaches to play on. and a riding stables next door if you like horses.
DP's all time favourite is Achmelvich, right in the North West of Scotland, right on a beautiful sandy beach.
we need to find ones in Devon, Cornwall, Dorset now, so please post your ideas for those.