How do you handle when other peoples children misbehave or become violent with yours?

My mother remarried and her husbands god children have emotional and developmental problems. No doubt caused by their parents lack of parenting and abuse. Its very difficult to attend family gatherings because they're always invited as well. Some of the problems that have occurred are the god children pulling my children under water and trying to hold them there when swimming. They also punch, use extreme profanity, throw things at other children including my own. I do make comments to them about this behavior, its not nice, someone could get hurt and so on. In the past when making a comment like the above their mother jumped down my throat about how I was saying she was a bad mother and it's not her fault due to the children playing rough with each other at home. I attempted to diffuse the situation and explained no I wasn't saying she is a bad mother merely that her children are being very rough and are going to hurt another child, period. I asked her if she couldn't handle it that she needed to keep them away from mine. I then found her bouncing her chest off of mine in front of a house full of guests and children screaming and threatening to beat me up! I then started getting my kids ready to leave while she followed me around attempting to intimidate me with her threats and jumping in front of me to try to start a fight. She ended up leaving before I did and I left shortly after. The entire scene from that day has me on edge every time a family get together comes up. Now i avoid most but have been catching heat from my family in doing so. My personal view is to talk about the situation and how it can be improved. This can not be done with this women. I hate to avoid family events but to me the stress and possible harm just isn't worth it. Oh, and my mother and step-father use the piss poor excuse that the god children have ADD and when not medicated act out. I myself have ADHD and was diagnosed as a child, my 7 year old son also has ADD without hyperactivity and neither of us have ever acted in a violent way when not medicated. Any suggestions on how to handle the situation would be very helpful!