How does "the child does not eat the apple" (Sg.) look?! The same?
I think that should be ok..?
सेब can mean, an apple, the apple, and apples...
"A child doesn't eat apples" should also be correct, shouldn't it? They said it's wrong. But I don't really think that in this particular case a/the makes any difference.
That would be
एक बच्चा सेब नेहीं खाना है।