Could this not also be बच्चा सेब नहीं खाता है?
It's always OK to include है
Doing so marks the answer as wrong for me, I've reported it
I think that's fine. I'm a native speaker from New Delhi, so I wouldn't know much about proper grammar though.
Should "बच्ची सेब नहीं खाती है" also be accepted?
As a learner, yes it should be.
How does one know that this is apples and not apple?
It could be the apple or an apple.
why is बच्चा सेब खाता नहीं wrong? english has "she does not love me" and "she loves me not" and although one is more common, I understand what is meant.
In Hindi it is not correct. Or, as you would imply, In Hindi it is correct not.
This in hindi बच्चा सेब नहीं खाता in english baccā sēba nahim khātā there is no है or हैं