My criteria for our kids were that it needs to be an economical pack (as they'll be outgrown several times), needs to be sturdy enough to put up with some rough treatment, be a dark colour, not too complicated, and have a short back size available, be relatively lightweight, and comfy for them.
My son (now 11) has a Black Wolf Mountain Ash 45L for Scouting
http://www.blackwolf.com.au/product/Mou ... ategory/31Our older Kathmandu Altai 50L is also an excellent pack for him - it's pretty light (1.35kg), robust enough for a kid to just sling down, not too fiddly, and a short harness lenghth. I think the 40L one is 1.2 kg. I'm not sure about the curent version of the Altai as we haven't tried it. You may be able to find one second hand.
http://www.litekamper.com/backpacks.htm - the Altai is the 10th down (I can't give you a direct link sorry because they don't make this version any more)
My daughter, (now 14) has used a BlackWolf Bugaboo 60L for Scouts and now Venturers - it may be a bit big for what you want.
http://www.blackwolf.com.au/product/Bug ... ategory/31The Black Wolf packs are sturdy, and not very expensive, but aren't in the lightweight category except for this version - which comes in 35L and 45L
http://www.blackwolf.com.au/product/B-L ... ategory/31. This doesn't have a proper padded hipbelt though, whcih is why we didnt consider it.