S.G.
I only breast fed for 6 months, but my 3 year old still does that. I treat it the same as hitting, playing too rough, pinching, etc. I say its not nice, and he'll be in time out if he doesn't stop. he can touch my arm, tummy, legs, etc., to explain what is nice, but its not nice to touch girls boobies and he will get in trouble. I would definitely apply the same rule to herself, and maybe not give it a fun name like "twiddling" call it what it is and tell her its not nice.