A little background before I get into this rant/plea for advice:
I have been with my man since before his children were born. They resulted from an extremely brief and casual relationship with a girl he knew from work. They are 5 years old now and we have a wonderful and loving relationship. I am a full time middle school teacher, and their father works in the restaurant industry. Their dad is wonderful and attentive, and their grandparents are very involved in their lives. Their mother has a history of abuse, violence, and generally unwholesome behavior that affects the children in many ways. She lives in a small 2 bedroom apartment with her mother, her sister, their boyfriends, her daughter from a previous relationship and our twins.

Most recently, the boys came back to us on Friday (We have a 2/2/5 schedule) acting nervous. I checked them over and found a bruise and a scratch on one of their faces. They are rough and tumble boys so I thought maybe he had banged into something as per usual so I asked, "What happened here? did you fall over?" his reply was, "No, Meema hit me there." Referring to his maternal grandmother who lives with them at their mother's house. Their maternal grandmother is already not allowed to watch her daughter alone because she was the victim of physical and sexual abuse under her care. For some reason the courts do not find it necessary to remove the children from that environment, but I digress.

I took pictures of the injury and informed my mother-in-law since my man was at work at the time. Contacting CPS or the cops was our first instinct, but in the past they tended to ignore the issue or call it inconclusive. We went to soccer the next day knowing their mother would be there. She generally keeps her distance from us but clings to the kids to the point of not letting them play when they want to.

The same child who had the bruise on his eye tripped and knocked into another child at the game during warm ups which shouldn't have been a big deal. I jogged over to see what was the matter and his shin guard had twisted and he said he wanted water, so i picked him up and began to carry him back to our chairs on the sidelines with the coach thanking me for taking him. The next thing i know he is being wrenched from my arms screeching like a banshee because he was in pain again. His mother yelled, "Let go! Give me my son, you bitch!" at this he began to sob and cling to my shirt. All i could get out was, "__________, Please!" before she managed to rip him off of me and drag him away. I had let go of him the second she grabbed him trying to make sure he was not hurt. I was so shocked I just stood there in the middle of the field for a second until my mother-in-law came to walk me off to the sidelines.

This is not the first time she has caused a scene in front of many other people and children, and I am sure it will not be the last, but I feel like it is my fault. Up until now I have stayed away from any even that she was to also attend, but as they get older and get more involved in sports and activities I want to be there to support them and help guide them. She seems to want the exact opposite of that. My stepson upon finally being let go from her 20 minutes later came back still crying saying that he didn't like that and that his mom wouldn't let him go even tho he wanted to come to me and his grandma.

If you've made it through this rant to this point I am really looking for advice, or alternatives as to what I could have done. I feel like I didn't do the right thing as a mandatory reporter, but at the same time I want to respect my SO's wishes when it comes to his children and he didn't want to call.