I was in this same situation a couple years ago. I decided that I didn’t have an expectation that my kids would haul extremely heavy loads (i.e., 60-100 lbs) due to their size and developing bodies. I also didn’t want them getting tired and wore out on hikes, and not enjoying the experience. I did have an expectation that they would carry their sleeping bags/pads, clothes, food, water, and small gear (i.e., flashlights, knife, etc.).
Based on my situation and after evaluating several pack options, I ended up considering more traditional backpacking packs (i.e., Gregory, Osprey, Deuter, etc.). These packs are relatively cheap, lightweight, durable, and have a lot of good features. Several models have harnesses that are highly adjustable, so the pack can grow with the kid until they are big enough for a more expensive hunting pack. We also backpack quite a bit in the summer with our llamas, so these packs serve a dual purpose. I ended up getting some 60-65L (3600-4000 ci) Deuter packs from STP for around $100-$120 each.
My 14-year old son packed out his bull elk hind quarter in his pack this fall. The pack worked great.