H.M.
I would get mall security's number on my cell phone and the second I saw her walk away in the future. I would call them and tell them she habitually does this and the child has gotten into trouble before when she has left him unattended. Including hitting your child. I'm sure they will come right down. Mall security is BORED! LOL! They are always looking for adventure. I'm sure she will be so mad she won't return and your problem will be solved.
Can't do much for the child unfortunately unless you call CPS on her. I don't ever usually recommend that though. Some people just don't think, because they were raised this way, that a child need supervision at all times. I'm not saying it's right. But they have a profound misunderstanding about how safe the world is for their child. Smaller community's don't even lock their front doors which is shocking to me. So she might be a fine mom and just didn't want to deal with the situation or you... and thinks the mall is safe.
I guess in this situation I would be most concerned about my child not getting hit again. That would have upset me to no extent. So sorry that happened. Me and my children go all the time to Oakview and Westroads and I've seen this sort of situation alot sadly. Although I am the crazy mom who follows her 3 year old around and makes her 5 year old stay by her so they don't have that sort of issue.