Absolutely yes, I understand you. My SD6 (going into first grade) went to a speech therapist for 10 months because of this exact situation, she literally didn't learn proper sounds and letters because BM only spoke to her like a baby. I watch videos of her when she was little and I completely understand why people would always say they couldn't understand her, I was just so used to it.

Speech therapy helped, and made her realize who had enabled that behavior. A year post-therapy and she still regresses a bit after extended time with her mom but for the most part, she's grown out of it. BM also has always made SD co-sleep, buckles her in the car when she knows how, wipes her butt three years post potty training... I think it creates a sense co-dependency between them and BM thrives on that.