Last year, my wife and I rode with our then 11 year old son (he turned 12 in WA) from Boston to Seattle on a modified version of the NT (we went across NY on the Erie Canal Trail crossed through Canada and then after MN dipped down through SD and WY to hit Natl Parks). Our son did great and loved it.
The first week we avg'd about 20 miles/day, 2nd week 30/day, 3rd week 40 or so/day. After that we avg'd 40 - 60/day and he did fine with that. In WA we stayed with a warmshowers family that rode the year before with kids on tandems. That worked for them, our son said no way would he want a tandem - he wanted to pedal the whole trip on his own power. My wife and I had trailers and carried most of the gear. We did give our son panniers to carry some stuff (mostly for his ego). Best wishes on your trip!