I have little to add, as Ben and Richard pretty much made my argument for me. First, the word “exploitation” is so incredibly subjective that its mere use negates the point of an objective argument. Second, talking about a “right to childhood” isn’t quite as subjective, but as Ben and Richard both pointed out, there are problems with both the ages during which a child is considered to be having a “childhood”, and what constitutes a “normal” childhood. And finally, anytime you try to base a real-world argument on a completely fictional story, it relies on way too many assumptions.
However, in the spirit of the questions…
Was Ender’s childhood stolen? No, he made choices to participate in the school voluntarily (this ignores the fact that his life was created as property of the state in the first place, which is a completely separate discussion) and thus no theft occurred.
Can exploitation of children be justified? ANYTHING can be justified at any time and for any reason. (The question is whether or not society and/or you as an individual accept the justification).
EXAMPLE: I have 2 kids. My little boy is four and he loves to help his dad. On almost a daily basis I will have him run little errands for me (things like go to the fridge and bring me a drink) – it qualifies as exploitation (I don’t pay him for it), and I justify it because he enjoys doing it. You can decide whether you accept my justification or not.