Honestly, I don't mind that either. It would have been too difficult to get 7 year-olds to play such complex roles as Ender, Bean, and the like, so they went with the next best option. The idea of very small children commanding armies is a stretch of the imagination already, but in the book you're inside Ender's head, and so it seems much more believable. To see it brought to life visually could be incredibly jarring to many viewers, especially those unused to Card's style and thought processes. Even beyond that, I can't think of any other actor that I'd rather see portray Ender than Asa Butterfield.