In reply to Nic:
Funnily enough, I've just spent the weekend camping with the Cubs at a trampoline park! Range of bags kids were in was vast!
My kids started out with Mountain Warehouse Apex Mini - will take them up to about age 8, depending on height. Not the warmest, but you can balance that with warmer PJs. Eldest now in a Vango Stratos 250 women's version, fished out of a bargain bin.
If you go for an adult bag, you can always tie the end off a bit to stop them slipping down into it and reduce the amount of bag they need to warm up.
Bear in mind that there's a good chance they'll get mucky/drinks spilled on them, so definitely go synthetic. Dread to think how much popcorn went home in them yesterday.
At the moment, a good starting point would be to look in Millets/Blacks sale at the Eurohike Adventurer bags as a budget option or Vango Sennan 250/Berghaus Transition 200 bags if you're going to use them a lot.
WRT Cub camp - worth considering getting them an over-sized stuff sack, so they can pack them themselves. I've got sore wrists from packing multiple sleeping bags on yesterday morning