I built a fly rod for my kids when they were 6 and 7. My belief is that kids of that age lack the hand size and wrist strength to cast a single hand rod of any reasonable size (>=7 feet). Perhaps even more important, they could not fight a fish because the hand strength is inadequate to keep a bend in the rod and pressure on the fish. So, I built a mini 2-hander of 9.5 feet, rated for a 4 wt.
The blank I used was 9 foot, and I added a bit of fiberglass rod scrap that the bottom cork would go on. This way, when they outgrew the usefulness of the 2-hand design in this size, I could just cut off the bottom cork, and a great 9-foot single hand rod would remain.
The rod worked great. The bottom cork was used for 2-hand casting and for a fighting butt, giving them the leverage they needed.