I think design number 2 is right on the money. The skegs are the way to go, provided that they are durable.

What kind of surface area are we looking at on the sail?

Having started off at that age with a sunfish, I think rigging more like a sunfish would make it more easier for a 10-12 year old.

Perhaps something with "reef points" on the sail? Then parents would have the option of powering the boat down for smaller kids.