I recently picked up an REI Stuff Travel Pack for my smallish 10-year-old (and will probably get another for my 7-year-old):
I saw these used for someone's child here on BPL. If you search REI for other children's backpacks, most have similar capacity (about 18-22L) but are heavier & more expensive because they have a few more zippered compartments. My 10-year-old could probably carry more weight in a bigger pack, but this pack should fit a non-bulky sleeping bag, clothing and a stuffed animal, which is enough. There's no hipbelt, but I don't think that's really necessary if you keep it light.