If you enjoyed this book and continue to read on in the series, you will quickly see just how "not childlike" these main character children really are. Now, on one hand, this is a fictional account of something that could likely never happen on such global proportions. But on the other, it is a social commentary on the fact that war, using children to fight in war, and training children to think like machines, will in fact take away their humanity and make them closer to machines. In this series, I do think it was necessary to use children to fight, but Card gets around the social dilemma by making his characters child "geniuses" who probably wouldn't have had normal childhoods nor lives anyway.
I think there is a sense in which in this novel that children are trained to be soldiers precisely because of the context. The trainers recognise the threat that buggers represent, and they also acknowledge that to train a force that can effectively destroy them would need a very special person, a commander whose training begins from a very early age. Using children therefore when they are at their most impressionable stage and can be moulded is a logical response to the threat.
At the end of Ender’s Game, Mazer Rakham explains that the perfect commander in the bugger wars had to be a child. He says that, compared to adults, children are less cautious and more willing to commit wholeheartedly to their goals. On a purely physical level, a child like Ender is faster than any old man could be.
In real life, and in the background throughout the book, most people think it is cruel to make children become soldiers. Fighting and killing are difficult and stressful. Soldiers, both adults and children, often suffer from guilt about what they do in battle. Also, children do not automatically know how the world works; their knowledge is shaped by adults. Because of this, children are easy to manipulate, as Ender is manipulated into killing an entire species.