My kids won't "go to bed when it's time"

Taken from the Skyrim Wiki:

After telling adopted children to go to bed, they will instead appear to sit on thin air or refuse to perform any other animations.

Can be fixed by simply saving and reloading.

I presume it's one of those bugs that have yet to be fixed, and unfortunately until then I don't think there's much more of a solution.

Save -> Reload -> They should be asleep upon re-load.