iirc baby talk can help a tiny bit with language acquisition but its far better to just talk normally, but yeah the talking down part is a massive part of infantalization. im disabled, mute and autistic, a few years ago for my partners birthday we went to the zoo (one of their favorite places), and this random lady while we were waiting for one of the other people in the polycule to go to the bathrom starts talking across me (as if im not there), about me to my partner, and then when she finally includes me in the conversation she goes "if you behave well maybe you'll get a toy" all baby talk like.
i cannot tell you how furious i was and the only reason why no confrontation happened is my partner is not one to terp me being rude (which is its own thing that they are better at now) and the lady didn't know any auslan.
Sounds rough.
FWIW, this was my first exposure to the word 'auslan'.