I started my son Tai on a 2m trainer when he was 9yo.
We then moved to a 5 line C-kite (4m), followed by another slightly larger C kite (6m).
This was flying in shallow water with me holding his harness.
The real progress came at 11yo when we moved to a 5m switchblade I, which he could fully depower and relaunch himself from deep water.
He was riding the board and getting upwind by about the 4th session. We did a fair bit of skurfing behind a dingy on his kiteboard too, to develop board skills.
He's now 13 and about 50kg, riding mainly on my SB3 8m, moving to a 6m above 20knots. He's progressing amazingly fast, now nailing a new unhooked trick almost every session, (and ridiculing his Dad's riding style.....that's teenagers for ya!).
Great fun to go and session together, calling each other into tricks and waves!
Train those kids safe and early, and they'll have an ocean sport to enjoy for the rest of their lives.
Andy.