speaking to a child at their level is the best way to keep them from speaking well. I never did it with my son and it didn't hold him back one bit. Everyone remarks his incredible vocabulary and language skills for his age. IMHO holding back with kids is an anti-pattern.
My aunt used baby talk with my cousin so much she accidentally invented a new language with him, and he ended up needing a bit of speech therapy to get back to a "standard" level of English for his age.
Perhaps coincidentally, he is now fluent in more languages than anyone else I personally know, and leveraged that into a consulting career.
