you either have to film the thing in pieces over a long course of years or change actors as the kids age.
They're pretty good at falsifying the age of the same actors for movies these days. See: Benjamin Button, A Beautiful Mind, etc. Doing it with kids would be new, but not impossible, and would not inherently suck.
Animated that's the way to go. Then you only need voice actors. Aging them is a piece of cake and the environments/special effects are all CGI. It could be done and it could be good.
I would absolutely love an animated Ender's Game movie. However, there's a pretty hefty subset of the world who wouldn't see it precisely because it's animated, no matter how awesome it is. I don't think OSC would go for that; I wouldn't in his place.