I'd probably say to them that it's for adults only or something if you don't want kids there, but like Carteki says, if you've got kids who are going, then may as well invite theirs too.
I've got kids but I don't feel offended if someone has an event and doesn't want kids there. I'd rather they did say though just to avoid confusion.
But if your child will be going to bed early, it'd probably be best to not invite their children too - many kids around here (both expat and local) don't go to bed until late when there's something like a BBQ going on (mine never did, they'd often be up until 11 when it finished ..........that's when our
condo kick you out of the BBQ pit

).