"He spends a lot of time with his children."
Translation:Han tillbringar mycket tid med barnen.
Swedish very often uses the definite form when English will use a possessive pronoun. If you say in English "I'm brushing my teeth", the best and most natural sounding Swedish translation is "Jag borstar tänderna". And the other way around, if you'd translate "Jag borstar tänderna" into English, the best translation would be "I'm brushing (or brush) my teeth", not the teeth. "Jag borstar mina tänder" is not wrong in Swedish, it's just not the idiomatic way of saying it. – In this particular case, both ways sound pretty natural, however I still think "Han tillbringar mycket tid med barnen" is a little better. You're right that the sentence does become a little ambiguous, but the first interpretation of it would still be that it's about his own kids.
Yes, but it doesn't mean the same: if you say sina barn, it is his own children. If you say hans barn, it's some other male's children.
If you say barnen, the standard interpretation will be that it is his own children. But it may also be some other group of children known from context. E.g. if he works in a daycare center.
I don't see how Han tillbringar mycket tid med barnen can only mean he spends a lot of time with his children. Who is her? Maybe he is a teacher, a daycare worker, a nurse/doctor in a children's hospital, a scouts leader, a trainer in a sports club for children etc.. Also, there is no context for who these children are and so I don't see how it could be clear that barnen could not be interpreted as the children in the hospital, the children in the school, the children in the club etc. I would have to agree with maxadrums here.
I did say "But it may also be some other group of children known from context. E.g. if he works in a daycare center." so I don't really see how you disagree with me. If he isn't known from context to spend time with some specific group of kids like that, the first interpretation will be that we're talking about his own children.
Maybe I am misunderstanding you and so apologies in advanxce. The translation of barnen is given as his children. I assume that his means that he is the father of the children and as an english speaker that is how I would interpret the phrase his children. Barnen could surely mean either his children (from the previous context not given) or that they are the children he is teaching etc. In order to specify that we mean his children I would think that sina barn would be more correct unless the previous context meant that barnen could only be interpreted as his children. So my point is that the english translation given by DL is more specific that the Swedish sentence implies from the limited context which we are given.
If you want to say in Swedish that he spends a lot of time with his (own) children, the most natural, idiomatic way of saying that is barnen. That's why it's the main translation, we wrote it that way on purpose to teach this. sina barn is also great, but it's really very natural to use barnen.