I'll have to agree with Adie, it all depends overall intent of pack. I've only made two packs, by no means an expert, but the best learning experience was to really think long term what I will want to get out of the pack. By that I mean required volume, desired packload, what "seasonal kit" I'll be carrying. There is information out there, but it's difficult to find. I studied a lot of different designs, and others trial and error (thanks DaveC). The two that I made, one was 42L and the other was a 58L, both were pocketless but had daisy chains for modularity. The 42L would be my spring/summer/fall and the 58L would be my late fall/winter/packraft pack. By knowing your pack intent, I think you can search the specific questions you may have in designing (i.e. should straps thickness, width, etc.)
Like most people will tell you, it starts with a suspension system. Like Adie, I'd recommend possible 1-1.5" larger than actual torso, it allows growth, and as long as you use load lifters you should be able to get a pretty good fit on a child. I've made three different sets of shoulder straps, although they've turned out great, I often think that ordering a complete set would save at least 25% of overall construction time.
Start with backpanel and what frame system you'll use. I've used everything from 0.040" aluminum with 2.5" holes for weight purposes, to coroplast, to 3/8" sturdy foam with .75" wide x .125" thick aluminum bar stock for frame. They all work, you just have to find what works best for you. Quite honestly my favorite might be a full length thermarest prolite.
After frame, determine what you will need to confortable carry that. Do you need thick or thin foam pads? Wide? Narrow? Next think of lumbar support and hip belt, how necessary are they and what thickness?
Once you've got that figured out, you're done with the hard part. Now to the fun part, design of looks, volume, pockets.