Here are some options
Kelty Sanitas
and
Kelty Jr Tioga

I have the older version of the Junior Tioga. It is lighter than the Sanitas. I have had my 8 yr old wear it with only his sleeping bag and clothes, and it was pushing his limit. It was good we weren't going far. These packs are perfect for 10-12 year olds. After that, they can take a larger pack. For kids younger than 8, I would say you need to carry 75-90% of their gear. When my kids are 4-6, they only carry water, snacks, and rain gear.
_________________________
I've taken a vow of poverty. To annoy me, send money.