In this special situation, I think the most idiomatic and culturally appropriate question to ask is simply:
How are the kids?
This phrasing is less intrusive. It does not make any assumptions or convey any expectations about whether or not your friend has had or taken advantage of an opportunity to see his children. It allows him to easily answer in any scenario, and allows him to choose how much information and detail he wants to share. He could answer:
- Great! I saw them last week...
- Unfortunately I haven't had a chance to see them lately.
- They're fine. (For whatever reason, he has chosen not to share further details.)
... and so forth. In many social situations such as this, English speakers (at least American and British) take special care not to ask questions that could be considered intrusive or that might put a person in the uncomfortable position of having to explain things or share details they would rather not divulge. This differs from some other cultures which are more comfortable with more direct conversations.