I have a 6 yo and a 9yo who like to mess around with wings and have ambitions to wing. 2.0 is a good size that they can handle pretty easily. If they start to really charge high winds, they will need a smaller wing but for now 2.0 seems good even for my little guys who is only 20kg
So far they are just riding SUP boards, I got them one of these Naish inflatables that were on sale cheap:
www.naish.com/products/s26-grom-inflatable?srsltid=AfmBOopszj85IloYKnR0LJYKJrtD1f5madV30HeOxgrv2o2XaJ2cqc11Other considerations with kids is that things like knowing which way the wind is blowing are not obvious and take time to learn.
My 9yo is doing sailing class in optimists at the local YC and he's learning a ton about how to read the wind, tacking, going upwind etc. I think next summer we'll put in some real time with the wing and/or windsurfing.
They also do a lot better as they get stronger, he started messing around windsurfing at 6-7 and while I could set him up with the rig and he could blow downwind (fun!) he wasn't really strong or determined enough to really do it.
Good luck and keep it fun!