What's the courteous way of saying, "I want to kiss your face"?

It's both embarassing and unbelievable.

How about "Kiss your face, ma'am"?

Every time you see them, you feel like you're on the cusp of laughter, but couldn't say what the humor was.

Unfortunately, there's no recourse. You can't tell them. It makes no sense. If you're lucky, they'll eventually find the same humor. Else, you become frustrated, and a little humiliated, as long as the reasons are unknown. And you'll miss them, as long as you can remember what happened.