I've never seen or heard of a mother cat batting her baby around like a ball and pouncing on it like it is prey. I would supervise this situation and when mama does it again, pick up the baby and tell mama, in a very stern voice, "NO".

Just a few questions for you.

Does she do it with only this one kitty or does she do it with the others also?

Is she still letting this kitten feed off of her?

Does the baby look sickly to you, compare to the others?

How old are the babies now?