I made tiny sleeping bags for my two when they were very little, but I bought snugbugs for them when they were primary school age. Summer use only, really not warm enough otherwise. On occasion I did use an adult one and as Mike said tucked it back up under itself to create a double thick bag for them.
If it's cold and you're out with them, share heat; pack two kids into one opened bag and drape the second one over them. Kids think it's fun and they're warmer…..even if they do fight with their brother
The stuffable padded jackets make good cover overs if a sleeping bag is a bit too thin. Easy to pack, comfortable to wear around camp, cosy tucked over them at night. There was a link not long since to the English company who makes them, and they were being sold very reasonably indeed……something knitwear…..if I find it, I'll post it.
M