Hats and socks keep me warm when camping in cold weather (as well as a good sleeping bag and a jumper). Your DDs probably wouldn't want to wear them though, and they'll probably cope very well and won't be as cold as you fear anyway.
Thermals. Nice warm hoody over pjs. Multiple socks. If there's several of them in each tent they can huddle together for warmth!
I'd send them dressed in a good warn 'base layer' of thick socks, long johns under jeans, vest under tops, and probably not expect them to remove that layer (that's what I used to do when camping!) bit stinky after a week but for 1 / 2 nights they'll be fine
Make sure they have waterproof shoes / boots as cold wet feet is the hardest to warm up from IME.
liners for their sleeping bags. Makes a big difference.
what are they sleeping on? airbeds are really cold to sleep on, roll mat or self inflating airbed better, no large volumes of cold air. also take an extra layer to go under the sleeping bag - blanket or similar.