Hi Gestalt
You should consider a special developed kid rig for your 10.5 year old?
Note that even if some other freestyle sails should be lighter, then they are developed with quite a 'high pull' in the sail (so it is easier load up and pop the board out of the water). This also translate to a bit more backhand pull and can fell 'heavier on the water'. Especially for a kid.
Also be aware of the boom cut out and the clew position might be to high for a kid.
There is a lot of good kid rigs out there from different brands.
Simmer Style is also coming with a few sizes especially developed for kids.
Lighter, lower boom cut out, lower clew, special short and lighter mast with the right softness
+ the kid boom is lighter with a reduced grip.
I had a 2.8 proto type here and they work and look great. I expect there will be a few bigger sizes as well.
I'm not sure when production is ready, but I can let you know when the word is out.