Another vote for the TNF Tigger sleeping bag. It's barely 2 lbs, compact and very warm. I would say a 5/6 yo can probably be expected to carry a small pack w/ Sleeping bag, sleeping pad and some clothes (for just an overnight: a change of sock, undies and a rain jacket). Plus a couple of fun items like a headlamp, stuffed animal. Also make them wear a whistle, and you should have one too.