I think that your description of "sexy" is a bit vague; and could lead to using "sexy" for any pleasurable visual experience.

I would start with your pretty good description of "genitals" and explain that when they feel good by being touched you get a tingly sensation.

I would incorporate this into "sexy." Which is that sometimes when people see someone or something it excites them the same way to look at that thing as the excitement they get when they touch themselves. "Sexy" is whatever gives our genitals a tingly sensation to look at.

And "Stripper?" A stripper is someone who removes their clothes while dancing for people who find it sexy and want to pay them for it.

Of course those durned kids have a way of asking questions "out of order" and derailing the train of thought. :)

Dante