I second the Dana and McHale ideas, but I have more thoughts for you. I also take my kids backpacking, and have found that I can keep my load mangeable by using ultralight gear for everybody. I made custom packs for my kids, so they can carry most if not all of their own stuff by about age 5. They started carrying just the stuffed animal at age 3 or 4, then just their sleeping bag and stuffed animal, and so on. By 6 or 7 they are carrying all their own gear, which measn a pack weight of about 5 or 6 pounds - no problem for them to carry. The kid size UL packs weigh about 12 oz with hipbelt and internal frame. The key is keeping everybody's gear light, and starting them out with a pack, no matter how light, on their very first trip. That way they get used to carrying a pack and you can add stuff to it as they grow. My older son is 14 this year, we are doing the JMT this summer, he will carry his full share of everything except the tent.
Oh, one more thing, I believe Gregory offers different size hipbelts for their packs, and some others may as well. But the packs are pretty heavy - that's the advantage of getting a McHale - you can have him build a big pack with a great suspension using light fabric and a simple bag, so it will be as light as possible. I'd expect you could get a pack that weighed about 3 pounds and carried those nig loads as well as anything you could buy.