"The children have their own bedrooms."
Translation:Børnene har deres egne soveværelser.
Sine is for one person owning multiple things. Here, you have multiple people owning multiple things.
The subject isn't the owner, it's the ownerS. Plural. Sin/sit/sine are for third person singular. This sentence is third person plural - which you can see because you can replace "The children" with "They".
Hi again :-) I think you can use sin/sit/sine for both singular and plural. The important thing is if the owner is subject of the sentence or not. If it is subject, sin/sit/sine should be used. If the owner is not subject of the sentence, you use hans/hendes/deres and only here, it is depending on the number and gender of the owner. At least that's how I understand this table: http://www.speakdanish.dk/en/grammar/pronoun-pers-table.php
Well, you might be right, but the notes here on Duolingo do not agree. They specifically say it is for third person singular, and I think that's why sine is not accepted in this sentence.
If you think that sin/sit/sine can be used to translate "their" as well as "his/hers/its" then you will have to take it up with the authors of the Duolingo course, because they will have to change the lesson on possessive pronouns.
However, I would note that I've found one more link that agrees with yours, but multiple other links that agree with Duolingo. In other words, I can find more sources that claim that table is wrong. I have sent that website a question asking them about it.
EDIT: Okay, I found out more. I couldn't get links to two pages on Google Books to work, but they both say the same thing: that some people use sin/sit/sine in the way that table describes, but that many Danes think this is wrong. It is seen by more traditional people as bad grammar.
Update: the person I contacted at speakdanish says they think the table is wrong and they plan to fix it.
http://www.speakdanish.dk/en/grammar/pronoun-pers-table.php on the last line, if the subject loves someone else's wife/husband/children then ''De elsker deres barn/børn'' (They love their child/children): if the subject loves his/her own wife, husband/children, then ''De elsker deres barn They love their child; De elsker deres børn; They love their children'' The adjective remains 'deres' when the subject is plural (they).] so does this mean , as some of the comments suggest 'they' is not included?