Boston's new obsession (train or bum bum?)

Boston's new favorite thing is watching choo choo trains (real and model) on YouTube. It's the first thing he asks to do in the morning and last thing we do before we go to bed. We will put the curser on the first video on the right side of the screen and all he has to do is click it when he wants to move on. So the other day and Miah related to me a funny story that happened while I was at work. Boston and Miah were sitting next to each other on the couch each on a laptop, Miah working, Boston watching Youtube when this happened

Boston: Dada, bum bum. Bum bum.

Dad looks over at the screen. One of the videos on the right side (amongst the very innocent videos of trains is a girl in a thong bikini). ??????????

Dad: No bum bum, Boston.

Dad tries to click on another video. Boston swipes dad's hand away.

Boston: No, my bum bum. MY bum bum.

Dad: No Boston, let's watch another video. How about choo choo trains.

Dad clicks on a train video.

Boston: I want bum bum. Dada, I want bum bum!!!

YIKES!!!!!!! I didn't know whether or not to think it was really cute or if I should be a little nervous.