In a nutshell:
DD1 and DD2 (6 and 4) in bath
I overhear DD1 telling DD2 that she is "going to suck her nipple"
I walk in and she is
Giggles all round from the pair of them
(I am BF DS all the time a lot and have my boob out on a pretty much permanent basis)
This is normal, isn't it (tell me YES it is)