I started out my 12 yr old last year with a 8'6' 5 wt St Croix. He was having problems with his backcast on the longer rod, not only catching every tree and bush on the creek but also whip snapping off flys. This year I handed him an old 7'6' 5wt Browning rod I had hanging on the rack. He's been doing a much better job casting it on the small streams here in SE PA. He doesn't seem to have as many problems with the shorter rod snapping off flies.

By the way if you don't currently tie flies, you better learn, otherwise your kids will bankrupt you. I tie up a minimum of 3 flies of every pattern to give my son we go fishing knowing he is going to decorate the trees with 2 out of every 3 by days end. I will also tell you that watching your son catch his first trout with a fly is the best experience a fly fisherman can have.