I've started taking my kids on overnight hikes and was looking for lightweight sleeping bags they could carry with decent temperature ratings and after much research discovered the Marmot Trestles 30. It weights 1080 grams, compresses down to just over 8 litres, and has a comfort rating of -1c. It suits kids smaller than 5ft/152cm. Reviews online are quite good. Some on Amazon complain about it not being warm enough, but it appears that the reviews are mixed up with reviews for a 40 degree f version.
I paid over $50 for this a couple of weeks back. I paired this with a Roman Airtrail R2.5 Self Inflating Mat - $29 delivered @ Fishing Tackle Shop and feel confident enough to use it for overnight hikes year round around most of the country. We plan on using it for a few hikes in Tas over the next month or so, as well as a couple this Winter where we'll be staying in huts.
How old are the kids? Are they carrying their own rucksacks?