Nothing! "What do the girls eat?" is a perfectly OK translation.
Furthermore, I would like to request the contributors not to sacrifice the context dependability of Hindi on the altar of a/an/the.
I see you doing Japanese & Korean -both are highly contextual – there the situation in which words are being uttered is more important. This implies that you mostly left the pronouns and other superfluous words, and like Hindi, there are no a, an, the.
So, to some extent, Hindi is similar to them in this aspect.
So, I suppose, in a later phase of this course, if they think of including stories, and think of a story about a PG Home where some girls come to stay. The cook there, who know that they have come, would most likely ask "लड़कियाँ क्या खाना पसंद करेंगी?" So, due to the basic nature of the language – its reliance on the context – he will not utter वो / वे before लड़कियाँ, and sentence will still be perfectly understood as "what will the girls like to eat".
I think I have made my point. :)
the girls is correct too. in another sentence लड़कियाँ पढती हैं। they give the correct translation as "the girls read". You have to go based on what is more proper in English as to whether or not the or a/an belong there. I also have never heard a native speaker say एक लड़की for "a girl" but just लड़की for "a girl".
I believe that "What do girls eat" is a general statement or question. It refers to every girl in existence. Where as "What do the girls eat" here might give an idea that the sentence is meant for a specific group of girls. If that's the case, "वे लड़कियाँ क्या खाती हैं?" Which can also be translated to "What do those girls eat?"