There are several points of argument for both the affirmative and the negative sides of this debate. For simplicities sake, I'll take the "in favor of" using children side.
In the context of the novel, it is really the only choice. The science fiction nature of this novel necessarily separates it from our world today--and allows us to consider the events in the novel as realistic for the context in which they are presented.
Consider first the reasons children are selected for Battle School. They are more moldable ("maleable" as Ender is called) and haven't created any bad habits yet. They probably also tend to fight and work hard with less explanation, as adults tend to become more suspicious of authority and jaded by age. Also, at this point in the future, the kids who are selected for battle school are genetically "weeded" (I don't want to say bred) to be geniuses.
The last point I'd make in favor of it being okay for children to be soldiers is that they don't actually fight in the real war. They are not at risk of dying. They are merely playing "games" on the simulator, completely ignorant of the reality of what is going on. Again, this was necessary to get them to take the kind of risks that Ender took. Had he known actual humans were out there--he would not have taken the necessary risks to win the war. An adult might not have been quite as persuaded by the aspect that this was all a bunch of "games" done in the name of training.
well since they used a computer-based program to "move" the fleet to destroy the aliens(or whatever they are called[i forgot]) i think it would be appropriate. no children harmed due to actual combat. i think.