You have to modify the users priveliges to my understanding, same way they do it in schools.
So make a user profile that is an admin, this one is yours, set a password so the kids can't get on it.
Make a second user profile that is a guest, this one is the kids, no password so they can get on it.
From the admin profile, set user priveleges, guest (or whatever you wanna call it...make it the kid's name, they love that) and remove printing.
At least, that's how you would do it in XP I believe, I've never had to do it and OSX can't be that different when it comes to stuff like this, maybe different names is all.