be on your bath behavior

It's a suggestion not too unlike to "be on your best behavior." In this instance it is a recommendation to keep it clean and not to be vulgar, well too vulgar.

Grandma: "Now listen children. I need you to be on your bath behavior in church today. No one is to spit or call the priest a child-molesting pedo again!"
Louie: "Can I at least spit in the holy water, Grandma?"
Grandma: "If I don't see it happen, then it didn't happen."