Acceptable: Bottom Crack????

I hesitate about whether to post this story, but it is too good to forget so here it goes:

Today Theo was walking around the table very purposefully looking for his pencil and saying 'where is my pencil? where is my pencil?' We all kind of ignored him because it just felt a bit dramatic. Then all of a sudden he pulls his pencil up from under the table and said, 'oh, here it is. It was right in my butt crack.'

Jack and I were so startled and surprised by Theo's 'joke' (with no discretion) that we laughed. I then quickly pulled it together and told Theo that that joke was inappropriate and unacceptable, and I didn't want to hear it again.

Theo got really serious and said, 'OK. I understand, but would it be more acceptable if I said bottom crack?'

Seriously. HE. IS. FIVE. YEARS. OLD.