Echoing everyone else's advice, but also....when you arrive, if you are planning to let your dc run off and play with the other kids, do set firm boundaries with her as to how far from the tent she is allowed to go - just general safety rules really. And make sure she can remember what your tent looks like!
We have a lamp that you charge up from the car's cigarette lighter and it lasted such a long time (ie several nights), really good. I think it was from Argos.
If you're camping on the coast, don't forget all your beach gear!
A windbreak or two is a fab idea too as we discovered last year, to put around your tent so you've got a bit of privacy and shelter if it's windy.
Oooh! Don't forget a mallet for knocking the tent pegs in!! Very important that one
You'll have a great time, it's loads of fun