That movie will really be hot. A tip, find a matte, if you have After Effects, you can key that color out. It's basically like photoshop, just the clever placement of screens will allow you to quickly make compositing easier through color keying.
I really would like to see this one, this is something i wanted to do with my nephews for some time now, neither will sit still long enough to capture the right footage. But i'm sure they would get a kick out of seeing duplicates of themselves. Maybe one day when my patience is better, 20 minutes with them... "Ok nephews, ya gotta go home."