name calling

my kids love to call names playfully. they hear my dh and i do it all the time to each other. they call things like cheese head and gumface. they dont mean anything, but occasionally it does turn mean...to poophead or stinkbutt. how do i limit the one and not the other? it doesn't make sense to me to try to enforce something like 'no names ever' when they have a good time, and its completely harmless. im trying to teach them how we shouldn't say things in anger. this takes hold a little with my 6 yo, but my 2 yo has no idea...