Ender's Game is like Family Guy, but dramatic and SF. Okay, the two are not much alike, but there is one trick the two have in common.
In Family Guy, we know Brian is a dog. We can see that he is furry and has a tail. However, he is so much like a human - he is bipedal, speaks English, dates human women, etc. - that it's easy to cast him as just a furry human. Then all of a sudden he starts chasing his tail or playing fetch, and we think, "Oh yeah! 'Cause he's actually a dog. HA!"
What's comedic in Family Guy is heartbreaking in Ender's Game. Ender and his peers are intelligent, learned and precocious, and their situations are those in which we would only place adults. As we're reading, it's easy to cast the characters as equal to young adults or just guys who take the opportunity to act a little less mature without many women around. Their actual age becomes unimportant to the story. But then they do something age appropriate - like cry themselves to sleep - and it hits you, "Oh yeah. 'Cause they're actually kids. God, that's awful." And you just want to go hug your own child and tell him to never join the army because killing is bad, and dying is worse.