What are they having? O que eles estão comendo e bebendo?

I think in informal ways it could be understood (so don´t be hard on them)
But it´s interesting to explain that it is more usual (with that structure):

What are they are having for breakfast/for dinner/for supper?

You could elaborate further, explaining that one could misunderstand (I mean the perception of ambiguity) and think that what they were eating at such meals.
To the latter - "what are they eating", to avoid ambiguity seems to memore to the point (and would be more specific in kind) - since the following question would be "what are they drinking? Or even what are they having to drink?

I mean the usual, natural way, at least for an ESL student. But I am not definite about it, others may think different or bring up more insights.
But, somehow I think a native would understand, don´t know if it sound natural.
But then, ESL-wise I would advise the "..for breakfast/dinner, etc", at least for the time being.

"What are they having?" is perfectly natural if you are asking a waiter or companion what someone else is eating, and you thought it looked good and might want to have it too, but you want more information about it before ordering it yourself.

And if you see something and immediately know you want it, you'll just say "I'll have whatever he's having!"

And if you arrived late to a group dinner where your friends already ordered you might ask for suggestions with: "What're y'all having?"

But without more context about the image in the book, I couldn't say whether more specification ("for breakfast") would be more natural.

Nicely put, Bryan. I could not have explained it any better!

"What are they having?" is perfectly natural if you are asking a waiter or companion what someone else is eating, and you thought it looked good and might want to have it too, but you want more information about it before ordering it yourself.

It was just what I thought at first, but the words didn´t come to me, thanks for your insight dealing with both - in a general way in a restaurant and then "the picture in a book" contexts.