And it must have sand, and not be only pebbles. I know you won't WANT to recommend such places you have stayed because you're probably about to book again, but you must. It is a moral responsibility to tell others about them, not keep them to yourself, please.
It mustn't be a caravan park. And ideally it is all on its own, away from the crowds. I do not care a hoot about ice cream shops, or discos, or evening entertainment, or supermarkets nearby. In fact, such things would put me off. The more wild the neighbours, the better. If it has the word 'luxury' in its description, even better, but we can rough it too.
Scotland, Wales, Ireland, Cornwall, Suffolk, Northumberland, wherever: really, I don't care. But my children's happiness this summer is depending on YOU... No pressure or anything.