When a child deliberately misunderstands what is said, it is obnoxious. Mrs Piggle-Wiggle, I believe, called it I-Thought-You-Said-itis and treated the afflicted by filling their mouths with frogs. Or something like that, it's been a while.
When a parent does it, however, it is perfectly acceptable corrective tool. A mild, playful reminder that when you talk as if your cheeks are stuffed with hard-boiled eggs no one can understand you.
Exhibit A, pretty much always:
Edward says, "Uhwnnrhhmchuzsangitch."
I reply, "I'm sorry? You want a hamster sandwich?"
Hilarity ensues and Edward then clarifies in rich yet dulcet tones that he would like a ham and cheese sandwich. Eventually this need for repetition will lead to self-modification and Edward's future peers, colleagues... all of humanity really... will thank me for the time and effort I put into getting my children to keep the E N U in enunciation.
Well, most of my children. Caroline always speaks like she's about to hand it over to Trevor who is reporting live from the scene and Patrick. Ah Patrick.
Exhibit B, yesterday, driving home:
Patrick says... god only knows what.
"Mister delicious said yes?" I offer.
"Yes," Patrick replies, coolly. "Mr Delicious Said Yes. That's exactly what I said. It's the title of a romance novel I'm writing."
"Glurk," I answer.
"Why? Why is it called that? Is that what you want to know? Because 'Mr Delicious Said No' wouldn't be a romance. It would be... a tragedy."
PS Today is my birthday. I! Love! My! Birthday! I'm not so crazy about the eye wrinkles and this new wibbly action that seems to be taking place under my chin but apart from that... birthday! Yay!