I have seen this latter part of the tape, too, and you're right; it isn't always shown. Sometimes it's cut off.
It is the most damning part of the tape that I've seen. The mom backs away as if she's finished, then she goes back in and proceeds to punch the living daylights of that little baby.
She has turned herself in, but she didn't have the baby -- either that or the child had already been whisked off to the hospital.