As OM stated, you should be looking for a pack with good hip belts. This will carry most of the weight. She should be carrying very little weight on her shoulders.

The Deuter Fox is a good, lightweight children's pack. It also has a hydration pocket inside.

<edit: just re-read your post and you don't say whether you are dayhiking or backpacking. Camelbak makes a mini-MULE kid's pack that carries a 1-L bladder and has around 500 cu inches of storage for other small items. It's a good hiking pack, both my kids share it alternately or fight over it, if we're all going out.>