Was the girl a willing participant in this activity? If not she may have some serious trauma issues from it. If she was I'd still call it abuse due to the ages, but I'd bet it would have lesser impact on shaping her future.

As far as what your asking, certainly he was probably curious. Most kids are, especially at that age where they're getting the talks. You just don't need to act out your curiosities with little kids. He was probably thinking "Hmm, how's all this work?" not "I am going to rape this little girl and steal her innocence".