My best advice is not for a specific brand or model, but rather that you go to REI (or some other Full-Service Outfitter if you have one near you) and have the kids professionally fitted for packs.
A good fitting should involve measuring the hip-to-shoulders distance, then trying a variety of packs WITH WEIGHT in them.
Now, on the one hand this seems a bit like overkill for children who (a) will have a limited selection of packs anyhow, (b) won't be carrying that much weight, and (c) will likely grow very quickly, but I still feel this is a worthwhile activity.
First, there's all the considerations about having a pack actually fit. Even after many years backpacking I will still sometimes select a pack for myself that I think fits only to be proven wrong either on-trail (Bad) or by an in-store measurement (Good). An ill-fitting pack with even just a few pounds in it will make the experience uncomfortable for the kids and could be enough of a turn-off to sour them on the experience.
Secondly, I've yet to see any kids NOT enjoy a test fitting, as long as the experience doesn't take forever. A good fitting should involve loading up the packs and then having the kids hike/march all over the store. Most seem to take it very seriously and really enjoy it!
If you absolutely MUST save every penny by shopping equipment online, etc. then I still recommend you do the fitting at a FS Brick & Mortar store, THEN do your bargain hunting. Although personally I try to reward service with my patronage...