I can't speak to your daycare situation. However, I suspect she is just experimenting a bit with sounds. Three was the age when my daughter started vocalizing randomly for lack of a better description - walking around the house making weird noises, screeching, moaning, etc. as part of her play. She also learned to talk in funny voices. I didn't think it was a big deal, but I did insist that when we were not playing, like at the dinner table, she had to talk in a big girl voice.
So, I guess I might consider if it is really regression, or maybe actually part of her development - is she really talking like a baby or is she just mimicing others? Sounds like the latter, which makes it a discipline issue. She can talk properly and is choosign not to because it is fun to talk like a baby. When she needs to communicate or be polite, just tell her, "no baby talk right now." Make sense?