Question

Why does a guy smile when he sees a little kid slap your butt?

Answer 1

It is not possible to determine why a specific individual might smile in response to a little kid slapping someone's butt without more context. People's reactions can vary due to a variety of reasons, including cultural norms, personal experiences, individual personalities, or even unintentional nervous reactions. It would be best to directly ask the person for clarification regarding their specific reaction.