In the full context of the novel, I think it is arguable that the main characters are realistic. First, when the children in Ender's launch group gather to leave Earth, they tend to do exactly what normal children would do. "The other boys were bouncing on their seats a little, poking and pushing, shouting" (29). Others, like Bernard and his gang, calm themselves and establish an identity by ganging together and picking on someone weaker.
Later, in the barracks, the boys are scared, mostly quiet, and don't know who to trust. They all feel immediately comfortable with Dap, the man who calls himself their "mom" while at Battle School. "Dap had a room full of friends. Frightened children were so easy to win" (40).
The first night in the barracks further proves these young boys are, at this point, still very realistic and believable. Dap comes in to check on them after lights out, and walks around, "touching a hand here, a forehead there. Where he went there was more crying, not less," (44). As the boys are shown kindness in this unfamiliar place, we are given a glimpse into their true fear and childishness.
By the end of the novel, it seems easy to argue that the main characters in Ender's Game are not realistic because of the way they think, interact with one another, and solve problems like adults. However, it is important to keep in mind that circumstances such as alien warfare, the threat of Earth being blown up, and technology more advanced than anything readers have seen, are also highly unrealistic.
These examples show that the kids who come to Battle School are very realistic as children when they first arrive. Those who take to the training well, however, are quickly changed, in body and in mind. They are forced to mature quickly as they are challenged to lead each other, make decisions, and problem solve. Also, keep in mind that they are geniuses. Card portrays realistic characters who show realistic growth and change in highly unrealistic circumstances.