Why is having a hull design that would be competitive with up to 75 Kg a problem?
The class would have a much larger appeal. Instead of dumbing down the platform, why not make the hull design with as high a performance level as possible. Reduce the size and simplify the rig for those with small children or those desiring lower performance. There could be as many as three different size rigs…one small simple rig for the 7-10year old children, a slightly more sophisticated mid size rig for larger children of 11-14 years of age, and a fully optimized rig for older teens and adults. I think parents would be more inclined to buy into a class that their son or daughter could make use of from grade school to high school and beyond with just a rig upgrade.

What novice sailor could not use the extra stability a slightly larger volume hull could offer? After all it is only twelve feet long regardless, and the weight penalty would be negligible. RG if you have the time it would be very enlightening if you could do a rendering of your current F-12 design side by side with your interpretation of an F-12 expanded to carry a crew weight of 75KG. With a visual of how small the difference would be maybe there would be less resistance to a 75Kg crew weight target.

Best Regards,
Bob