I think it's fairly normal behavior. The poor little girl sees this kind of thing all the time, and is probably too young to understand what it really means. Pictures like this are everywhere: billboards, magazines... there were probably some hanging in the train station, selling who-knows-what.

Mom needs to help the kids understand there's a place and time - and an age - for this kind of thing. Including a warning about cell phone cameras and the internet. These are very real dangers to young girls.