My son is 6 years 8 months and I just built up a 26" bike for him, he is tall but not that tall for his age. The frame is a 14" Malvern Start XCS 7.0 I had a few nice parts lying around (light wheels, SID forks, XT cranks, SRAM X7 grip shift, carbon bars etc.). I was going to lace up some 24" wheels if it was needed but he seemed fine on it. He rode the dirt track at Sutherland track fine.
It was his first encounter with gears too; 3 x 9 as we rode I just shouted the numbers he should shift to, he'll eventually work out what he wants to be in but I thought I'd make it simple to begin with (leave it in the middle ring and just shift the rear). He was pretty stoked.
Never tried X7 before, the rear derailleur is fine and so is the grip shift for the rear the front shift has no intermediate clicks for trimming the chain. Which means a bit of chain rub on the front cage as the chain gets to far up or down.