He was a skinny kid, weighed maybe 55lb soaking wet. We had trouble finding a "real" backpack with a waist belt that fit. We got him the Deuter Fox30, which was just the right size, but the fully-cinched waist belt would invariably slide right over his non-existent hips.
My son is skinny, too. Rather than sewing on padding I just tape some blue foam on the straps. Very easy to remove when I use the pack.
Got it! And you're welcome. Looks like topshot has the other approach, for those who can't sew, or if you need to switch back and forth between skinny and less skinny users.
For the record, at age 10, Eldest Son weighed 55 lbs fully clothed, and was 50.5 inches tall. That year I think he carried all his own gear except the sleeping pad (too bulky), but very little else--just water and snacks. Probably about 10 lbs; I always tried to keep the boys' packs down in the 15% of body weight range. As a general rule, that kept the packs from being a burden and helped them have fun.
Day hikes of increasing mileage and increasing BP weight are great for kids to build up their endurance. Even though your child may be strong from dancing, hiking requires different muscles and is a totally different exercise. Training up for a hike can be half the fun of it. It will make the training more like a step to reach a goal rather than a drudgery. Also, freeze dried meals sound novel, but eating them can take some adjustment lest one suffers from gastrointestinal reactions. That would be no fun for anyone in the back country. Basically, preparation is the key to success. There should be no surprises about how gear works, how food tastes, or how the feet or legs feel. Keep us posted.