Wasn't going to touch this one, but thought this might help. Yes, this is a rare behavior on the part of a dog, but not completely unheard of with an intact male. ( An unneutered 1 yr old GSD left alone with a 3 yr old child in the living room of the parents home, comes to mind. A story here, several years ago) It does happen and it is not an aggressive act on the dogs part. They mount children all of the time. Most owners assume this is a playful or dominant gesture. The right circumstances, lack of adult supervision there to protect the child and determine what is "playful" and what is definitely not, a strong sexual drive on the dogs part and it's a tragedy in the making. People don't get it... never, never leave a baby or young child alone with a dog. This is so sad.