"Hvem sitt barn er det?" is just fine, and probably the best solution here. In speech you could also say "deres eller deres" and point to indicate the intended interpretation, but in many cases this won't be an issue in speech, because most dialects distinguish between the two (in mine, dåkkers = yours, dæmmers = theirs)
Yes and no. Short and common words such as personal pronouns are often the ones that undergo the least predictable changes.
For verbs and regular nouns, it's usually more about the endings, so once you crack the code for one verb the rest become equally obvious. The differences aren't as drastic as they might seem at first listen, and as active language learners you'll be used to looking for those sorts of patterns already.
"Is that child yours?" (plural)
"Er det barnet deres?"
"Is that child yours?" (singular)
"Er det barnet ditt?"
The reason that this can be doubly confusing is that English fails to distinguish between the 2nd person plural and 2nd person singular of the possessive, while Norwegian fails to distinguish between the 2nd person plural and the 3rd person plural.
yours = ditt, deres
deres = yours (pl), theirs
Context is your friend in both cases. :)
Then there is the older English, singular version of "thy" possessive adjective and "thine" possessive pronoun for singular "thou" nominative (thee object form) which is still found in old literature and prayers. You, which is now used for both singular and plural, was originally the plural form. That is why "you all" will never be accepted everywhere in the U.S. It is definitely established in certain areas, but it did originate from immigrants who had trouble with the one "you" system. If they had understood that "you" was originally the plural form, I wonder if people would have gone back to using the singular form?
"deres" is both used for "your (plural!)" and "their". You can find the forms in the Tips and Notes on the bottom of the page: https://www.duolingo.com/skill/nb/Possessives/tips-and-notes