I think this mostly depends on their size and capabilities. I will be taking 4 of my kids on several trips this summer. My 12, 13, and 15 year old boys, I can manage fairly well. My 9 year old daughter had me looking around at gear, she istall for her age 4'5" and is an athletic built 69#. My goal is to keep her around the 12# mark total weight.
One item I am surprised no one mention is golite. with my daughters measurements 26" waist and 15.5" torso, the golite jam small will fit her well with room to grow. ( I had her try on my wife's medium to see how big the bag itself was (50 L). I will get her the 35L, should be the perfect size for her. With only a 12# (or even 15#) load I am not concerned about the frameless pack, with the hip belt it still beats a rucksack and it is reasonable at $100, she can use it a couple years and then my youngest son can inherit it.