Are you saying that that this much sail area isn't safe enough or that the boat will be so fast it will scare the kids away?

I would have thought that the point of a multihull is to go faster. It's true that a certain amount of speed may be intimidating to a youngster, even if it is quite safe, but of course you can fix that problem by just buying an Optimist. I'm not sure that making a multihull more like the Optimist is the right solution.