It could be a reasonable question depending on the conversation. For example, if we were talking about a restaurant or the school cafeteria and I asked, the answer probably would not be "All of them." It could be a specific number during a certain period of time, or a percentage of all the girls. It could even be "None!" (Because they do not like our menu, or because they eat at the new place/in their house, etc.)
To make sense in English you would need another qualifier, like "How many girls eat here?" or "How many girls eat between noon and 1:00?" With no other qualifiers, asking "Does the girl eat?" means you are asking whether the girl eats at all, ever. So, of course all girls eat, unless they are starving themselves.
I think the question is posed this way in order for us to understand the translation more clearly. Yes it doesn't make total sense in English to say it this way, but for the purpose of learning Italian it is easier not to have the added complexity of the word "are" in the sentence which is not the focus of this excercise.
@LindaLee - Not exactly. Stanno is the plural form of "stare" and stare nearly always acts as the auxiliary verb with the gerundio form of the main verb. (There are some uses of the gerundio that don't take stare as an auxiliary)
"Stanno mangiando" = they are eating, and they are doing it right now, as we watch
"Sto bevendo" = I am in the process drinking, see me lift my cup
"Sta camminando" = He/She/It is walking and the action is still happening at this point in time
Hope this helps.
This is a difficult question - in English 'kids' is no specific gender. In Italian you have to specify gender. I believe that the "default" is masculine, so if you have a mixed group, you would say 'ragazzi'. If the sentence says 'ragazze', then it is necessarily specifically a group of girls.