I agree, a full size Start probably will not get them planing with the size sails they can handle.
Have you looked at the Pro Kids GO (100 liters 217cm long x 76.5 cm wide) ?
That might be a good choice as would the std. GO 122.
Have they transitioned from using the center fin on you Start? This will be a consideration as
they will need to learn to rail the board a bit to stay upwind.
Or you might find an older small pro kids formula.
The formula board will be more fragile, but they plain pretty early with smaller rigs.
Hope this helps,