I would of used "Hun kinderen zijn aan het spelen" to say "Their children are playing." To use "Hun kinderen spelen" would translate more to "Their children plays." At least that's what I think, I'm not Dutch though. Any feedback appreciated.
You are not wrong, however, in Dutch you can translate 'hun kinderen spelen' to 'their children play' as well as to 'their children are playing'. It's because the continious is not used in Dutch like in English, and used waaaaayyy less. :)