There have been a few questions with regard to what age and kit for youngsters starting out, this is my grandsons (8) set up which he handles well.
Rod
Shakespeare Odyssey 7ft 4-5 Brook Fly Rod | eBay
Reel
Avalanche Reel - Fly Fishing Equipment Reels - FastMail Tackle
Line 5wt
RT ENERGIZER FLY LINE - Ron Thompson - FastMail Tackle
To start 6 foot leader of 8 to 10lb and a un-weighted fly like a hopper, hares ear or ptn
then its time and Patience with them.
Josh was 7 when he started in earnest but hes a strong lad so some smaller kids may need to be 9+.
Don't over do it with them in the early days as there arms are not as strong as ours and tire quicker, but will soon build up.
Mike