I think it's awesome if your kids want to get into kiting at an early stage. I have been teaching a few kids myself (12, 14, 16 y/o) and have seen lessons with a 9 year old. I don't think age is the problem but I believe it would be a good idea to get them some lessons so they can understand the safety systems back to front and perform self-recues in deep water situations, as well as body dragging upwind and board recovery, also in deep water. Given their experience, I think one lesson will suffice. Would also recommend helmets and life vest at all times.
Also it would be a good idea to have them watch the latest beginner progression DVD, available at most kite shops. One of the best kites for kids to learn on is the Ozone Uno, it's an inflatable trainer kite.
ozonekites.com/products/water-kitesBasically it's the exact same setup as any inflatable kite, 4 lines and the ability to depower, etc. With this, you can have them work on water relaunching, kite stability and control, and the sweet spot on the bar. They over-sheet easily due to their small size, so it's great to teach anyone about how to prevent and react upon over sheeting...
If you're interested in more info, please PM message me.
Cheers,
Chris