Not sure how this fetish would be called.
In this doujin, a girl gets blackmailed and made to write like a little child (amongst other things). She's told to write exclusively in hiragana., with the master(s) on the other side of the web making fun of that.

I'm not sure how this would translate into English.
For Spanish, French, etc it would be writing disregarding accents; in German it would be writing all in small letters (IIRC you have to capitalize a lot of words in German for proper grammar and ortography).

But in English, how would you convey this?

Also the manga itself is about turning a school teacher into petgirl.