You specify additional information in a question when you think it might be unclear without it.
It is perfectly valid to ask: "What are you speaking?" Most English speakers would understand that to mean that you want to know the language. I suppose it would not be shocking if someone replied, "The soliloquy from the third act of Shakespeare's Macbeth". But that would be unlikely, as if we expected an answer other than a language, we would more likely ask, "What are you saying?"
Your second example is a more useful one. You could ask, "What are the children playing?" There are things that children could play other than sports, so someone might very reasonably answer, "They are playing Monopoly" or "They are playing that they are arctic explorers". But if you say, "What sports are the children playing?", now we know that those answers are not what you are looking for, you want to know whether they are playing football or baseball or tennis or whatever.
The issue is not that some categories of things you might ask about require additional words and others do not, but rather that in some contexts, you must add additional words to make clear what sort of answer you are looking for, while in other context it is obvious without you having to say. In other contexts still your question may be intended to be so general that almost anything would be a valid response. That gets back to your first example. If you ask, "What are you doing?" the answer could be the physical action that the person is performing at this exact moment, like "I'm sitting in this chair", or it could be some larger plan that the person has, like "I'm trying to get a promotion" or "I'm planning my divorce".
