“looked seriously at the MLD Kids' Prophet …all the weight will be on the kid's shoulders.”

Hmm? Not what I see. It appears to be all on the hips with the shoulder straps used as balance. I’ve used these type of backpacks before (using sleeping pad as frame and no padding in hipbelt) and I was surprised how the hips didn’t hurt. That’s the style my youngest daughter used for 3 years (with 8 lbs total load weight).

The small Mariposa also looks like a good idea. I have more comfortable results taking the carbon stays out and just using the sit pad as a frame.
http://www.gossamergear.com/cgi-bin/gossamergear/2007_Mariposa-Plus.html

-Barry