"What did you did?"

[Stefanie]

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

Ruth, satisfied with the skinned-knee stor and with that
photograph, was now looking over her father’s shoulder; something in the bathroom had caught her attention.
‘Where are the feet?’ the four-year-old aked.
'What feet, Ruthie?"
Eddie was already moving to block their view of the bathroom.
‘What did you did?’ Ruth asked Eddie. ‘What happened to the feet?’

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…

[Mister_Micawber]

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

[Stefanie]

OK, thank you!