I'm thinking of giving the little girl a pat on the butt.
What is your reason for patting her butt.
I am actually pretty excited about doing so.
Touching a child on the butt or other private areas is very inappropriate and it shouldn't make you excited. You should probably talk to a professional if you're having these types of feelings because they are not normal or healthy.
Duh, I'm only doing it for fun since she's a kid.
Whether it is for fun or not it is not a good idea to pat a little girl on the butt. This would be an issue for the parents and may also be something that the little girl would think was a problem as well.