Good luck. I haven't been able to find a supportive pack for kids in which the pack doesn't weigh 2.5-3 lbs.

One of the biggest complaints from young children is that the pack straps put too much pressure on their shoulders. A pack with a good hip belt and load lifters really helps this problem. Unfortunately, only a few of these are made and they are, as I said, heavy! It's a shame when half the recommended weight for the average 7-year-old is the pack itself!

Definitely do not use her school backpack unless it has a good supportive hip belt!

Yes, do watch what she brings!

EDIT: There's quite a bit about kids' packs in the "Hiking with Kids" section of this forum.