It is a fact that kids will not be able to retain body heat as easily as grown adults. It's all about the mass to surface area ratio.

That said, Rivers West makes a fleece blanket out of their proprietary H2P material that is windproof, waterproof, and darned near indestructible. I got one for "The Wife" and she calls it her pacifier because 5 minutes after she climbs in her chair with it, she is out for the count. They are lightweight and moderately compressible, but by far the best fleece product you will find.

There are a number of really good down and synthetic bags for kids out there. Check REI and Coleman, among others. Some of them stuff down to quite a small volume, yet when unpacked loft up very well. I suggest synthetic instead of down, given the nocturnal tendencies of many youngsters.

A four pound qualofil bag cut to a kid's size ought to be good down to minus 20, and should easily stuff down to less than 1 cubic foot.