My son has a 13.5" torso, 20.5" waist and carrys 15.5 lbs (including pack) when hiking in "self supported" mode. (For longer trips, I carry our shared shelter & kitchen to get him to 12-15% of his 80 lbs). We tried all the packs at REI, EMS, and Hudson Trail. The Granite Gear Vapor Trail worked best for my son - by a large margin (short torso, small belt size). The REI womens flash 50 and Osprey talon's were the best runners-up. We didn't try the ULA, SMD, MLD or Golite packs though...
The biggest issue was finding a pack with a short enough waist belt. The GG VT small belt was the ONLY ones that got small enough to work under load. The flash & Osprey talon came close, but slipped when loaded after a little while. Having the belt padding end 1.5 inches farther away from the buckle would have allowed enough tightening to fixed the issue...
We also found that adjustable internal frame packs didn't work for him. The shoulder straps adjust toward the waist belt to account for torso length - but the pack bag is still just as tall - so now it pulls him off balance. Some external frame packs worked - but only the ones where the frame telescoped to get shorter overall, and where you could also move the pack bag down so that the top of the bag was no higher than the top of his neck.
We found 50 liters was the right size to easily hold all his stuff, but not too much to carry. This includes the 48" ridge rest coiled inside the pack during 3 season use, or strapped to the rear during winter to make more room inside for puffy insulation layers. My son said strapping the CCF pad across the top of the pack felt top-heavy and pulled him around slightly when walking up stairs - but that strapping it to the rear was comfortable and didn't feel like it was pulling him backwards or causing him to lean forward any extra amount.
The combination of the GG VT's foam packpad (stiffened a bit with a laminated plastic backing), and the great compression straps on the pack allowed him to cinch the load into 1 solid mass - at least well enough to take ALL the weight off his shoulders and put it on his hips. The shoulder & hip padding on the VT is also lot cushier than the REI Flash, and noticeably cushier than the Osprey talon.
The downsides of the VT were that the torso was not adjustable so you'll be buying another 2 years from now, there was no big mesh rear pocket for wet rain gear, and the extension collar is annoyingly long. The long extension collar can be shortened in 5 minutes using a home sewing machine, rain gear can be stuffed into the side pockets (but then you lose a convenient place to store "small stuff you may want throughout the day"), and trading the rear pocket for straps to attach the pad may be better for many kids anyway.
The 100 denier fabric sides have been durabile enough so far - but it's obvious you have to be careful around tree branches & setting it down on rocks without sliding it (and he is). I'll probably replace it with a pack made of all 210 denier fabric (VT Flash, ULA, SMD, MLD, Golite) when he grows out of it, just so he doesn't have to worry.
ps: For trip planning; I find estimating a 2 mph hiking speed and two 4 hour hiking sessions (with a 1 hour lunch break) is relatively accurate for 11 year olds on rugged trails (lots of up & down along the ridge on trails full of stair-shin high rocks). 1.75 mph on super rugged trails (ie: hills as steep as stairs, and with lots of rocks). 2.35 mph on smooth trails with small rolling hills.