Right now based on what parents and kids are telling me, I'd say that the only person who wants the round bilge design is me <img src="http://www.catsailor.com/forums/images/graemlins/smile.gif" alt="" />
So, if I follow exactly what those interested want (not counting those here), its a hard chine design, with boards, that can fit an age range from 7-8 thru 16+, that can accept either rig, although the majority appear to want a stayed rig.
To be fair, most of the kids Ive spoken to come from sailing families, most own or have sailed opti's (and are really bored with it) and one or more of the parents sail multihulls.
Just from my personal perspective, I see the ability to swap from one rig type to another as a design plus that gives the designs more utility over the age range and beyond, perhaps the easiest way to deal with it is just cap it around some age limit?
RG