"What did you did?"

I’m just reading a book by John Irving, “A widow for one year” - and I came across the above phrase at least twice.
Here’s one example for how he used it

Does anybody know whether Irving had an intention using that phrase? Is it kinda slang? Or just an allusion to how a four-year-old speaks? Or…?
I’ve asked a native speaker from America (California) as well as a native from England and both have never heard…

Yes, it is the representation of a typical 4-year-old learning her grammar.

OK, thank you! :slight_smile: