"The bread is food."
From what I can remember. 니다 is often to say am; is; are. Korean does not follow the same guidelines to grammar. I do not know the exact structure but their's is different. But basically you are literally said Bread Food is in Korean. But because of the structure it would mean it would be Bread is food. Don't qoute me on all of this but 니다 means is.
Just to be clear, the sentence you provided contains a few typos. I believe you probably mean: 남자아이의 사과는 음식입니다. Right?
At any rate, both 빵이 음식입니다 and 빵은 음식입니다 can be translated as "Bread is food" (and Duolingo should accept both translations). The use cases are a little different, though.
The most concise way to put it, as far as this example, would be that 빵이 음식입니다 makes more sense as a response to "What is food?" whereas 빵은 음식입니다 would be a more neutral way to say that bread, as opposed to other things, is a food. I highly recommend watching the TTMIK video on topic versus subject markers; it's not intuitive to non-native speakers but does eventually start to sink in! Unfortunately, Duolingo is about the worst place to practice this particular distinction.