last weekend I took my eldest (just 6) camping in Derbyshire and dont worry too much about cold weather - other parents seem to think it slightly irresponsible
We have decent sleeping bags/kit, self inflating mats, insulated inner tent floor etc so dont see an issue myself. One reason I am ok (and this will sound the wrong way round) is my eldest actually feels the cold. If she gets chilled she wakes up, whereas my youngest (3) can be as cold as ice and shivering and still claim to be warm (?!)