Thank you for taking the time to explain it to me! I can't respond beyond the last comment, so I'll just do it here. This may seem like irrelevant information at this point considering my low level, but it helps me a lot to know just which parts are 'don't think about this, just use it like that' and which parts are 'you should understand this'.
I appreciate the input and information. :)
Completely different words are used in singular and plural. The word ребёнок has no plural form (ребята may be related but the meanings are different). Дети (note: not детн) is only plural and has no singular form but parallels the meaning of ребёнок.
Olimo mentioned человек and люди because they are a similar pair - parallel meanings, but человек is only singular and люди is only plural.
I can accept that as something native speakers would simply know, but being so far removed from one, I'd appreciate if you could explain what what, exactly, the difference in meaning is between ребята and ребёнок. If that's possible, anyway. Nuance is hard to pin down, I'll admit.
The second part of your comment makes complete sense, so I can accept that at face value. :)
If you want to know if they have children, then you would naturally say it using the plural children. But if you want to know if they have a child, then you would use the singular child. That's what the question is here. Of course, in most cases it's more normal to use the plural rather than making any assumptions about the number of kids, but there could be exceptions and this is apparently one of them.
It occurred to me, too, that, if you are talking to a stranger and you want to know whether they have any children, you would be much more likely to say 'do you have children' than 'do you have a child'.
It is not inconceivable that I might one day find myself in a situation where I specifically need to ask "Do you have a child?" rather than "Do you have children?" But, given that Russian has no definite or indefinite article, and given that we have no context, is "Do you have the child?" not a much less perverse translation?
Please contribute. You may notice something a native speaker missed. Grammar can be studied by anyone.
Experiencing a language helps. But nothing about language is absolute. So please put those questions out there.
Language evolves as we use it, too.
Homework: look up the definition of the English word "mark".
I just have realized how much easier is Russian in comparing of English, so many words in Englisch frazes. There´s a an The and so many unecessities we do not use. My birth language is kind of similar to Russian, yet very different, like the alphabet, but maybe it´s because I was learning Russian as a child.
Each person's duolingo computer is in their own language so though my tests are in "English" and Russian here, another learner here has a keyboard in Arabic, Chinese, French, Spanish... etc. right? Or is everyone learning English and Russian at the same time? Just wondering? Stupid question, just being nosey while overdosing on кофе.