Train the kid to tell them, "As soon as mommy and daddy can find one. They already bought the duct tape and cage."
I think that people think they're being friendly and don't realize the impact that their questions have.
I have three adopted children. The birthmother of my oldest, when she was pregnant with him, used to get tired of all the strangers who would come up to her and ask, "When are you having the baby? Is it going to be a boy or a girl? What are you going to name him/her?"
Finally, she started answering back, "Baby? What baby?"
That shut them up.
I was working on something similar, but your response nails it! Well done.
I know you said it in jest, but your post really hurt my heart.