Dermot,
I think the answer is to come up with a super simple design to build. This would also be super cheap to build.
The quicker and easier and cheaper it can be built the more likely it will work to bring people/children into the sport.

I think quick, easy and cheap much more important than performance for the first timers.

Now if they are silly enough to come back to build another they will have some building confidence and sailing experience and hopefully both were pleasant on the first attempt and will want something much more refined.

Now most of us have had pleanty of experience with numerous boat designs and have developed a more refined taste that would not be satisfied with what a first timer would want.

To make something like this happen I think you need to get people together to do this as a social activity so they can be built in groups/numbers. Sailing clubs may be a fertile ground/venue. The clubs could run some adds and manage the building projects.
You could pitch it "dads and mums come and learn some boat building and provide a thrill machine for your young children".

Regards,
Phill