I have put a stop to it with my kids....

My cousin breastfed and let her DD play with the other breast.... she kept doing it years after being fed, anywhere and everywhere. She would just put her hand down her top and pinch. It did look wierd.
But she now does it to anyone. And I find that gross. She has done it me when I was feeding H, and mum just laughed. I dont see how a 3 yr old groping other women is right??? She grabbed my DD, who has developing little breats buds, and it hurt. Mum said nothing, she thinks its fine.

To me, personally, I dont think teaching children that touching private parts is ok on other people. So I just dont let it start, then have to try and stop it later on when it starts to get a little weird.
Had I not seen this type of nipple playing, it probably wouldn't bother me, but I have, so it does.