I am sorry to see that you have posted this before on the Pro kids forum and had no help.
I do not sail starboards but I am a windsurf trainer.
For freestyle the flare is perfect.
I would recomend either the 88L or 99L as they will give you enough volume to do the none planning maneuvers like the gecko. As well as being realy good in the light winds. You will also be able to use smaller sails because you have more volume.
The 99L may be a better bet as you may grow and put on weight pritty soon.
Hope this helps.