"Do you have kids?"
Translation:У тебя есть дети?
The t-v distinction works differently in every language. I'm not proficient in Russian but I've heard enough Russian dialog and it seems to me that you would use вас when you'd call someone "sir" in English. So, probably not when meeting an acquaintance who is your social peer.
"Y" on it's own is meaningless I think, together with other words that represent someone you are talking about like "you"->"тебя" it's "you have".. if you really want to translate word by word to get the sense of it, it's like "at your ... something", "at his" , "at hers"..
That Serbian «у» probably corresponds to the Russian «в».
Russian «у» means 'near, at'; «у меня́» can also mean 'at my place'. It doesn't usually mean «in».
У тебя есть дети not working as a solution when input w/ roman characters (neither 'U tebya est' deti' nor 'U tebya est deti' are marked as correct) - this seems to be a common fault in the grading of questions using есть to indicate possession (the Roman transliteration appears to work correctly in all other cases I've tested so far)
ignore the ' I got a lot of answers wrong using it. You could also install free Google Input Tools - you transliterate something, ie write it in Roman letters, and you can see and select the Cyrillic version. Saves the headache of juggling when Duolingo will accept the apostrophe or not