In reply to girlymonkey:
I'll echo what others have said about kids at 11 being very different to each other.
I started taking each of mine into the hills "proper" at about 6 (the age they each did Scafell Pike for the first time). By 11 they were all doing long wild-camping weekend trips with me, usually in mountainous terrain.
I did keep their personal loads down to 6-8 kg in total, by carrying everything but their sleeping bags, waterproofs, food and water myself, which was a heavy load for me with tent and overnight food.
I do remember carrying heavy loads with crap rucksacks at about that age in scouts, but I do also remember several friends at the time straining and injuring themselve....I think we know a bit more about the mechanics of growing bodies now than we did 50 years ago, so no need to repeat the mistakes of the past on an "it were good enough for me when I were a kid" basis.
But as you're aware of the situation now, you can plan for it, check the youngest's loads, be ruthless about knocking back unnecessary items and distribute necessary heavies amongst the others/ yourself.
I've taken the odd group of youngsters out myself, and I do recommend a FULL kit inspection....I've seen 12 year old girls with full makeup bags in their rucksacks....not to mention 2-3 pairs of "smart" trousers and tops for a wild camp weekend!