Was I overreacting for calling the cops on my MILs boyfriend

I have never liked my MIL. Mainly because of what she put my husband through as a child. She is a woman who is afraid to be alone and will put any man before her children. She had really hurt him but they had made a mends and tried to rebuild their friendship. Unfortunately my husband passed away 6 years ago from falling asleep at the wheel. I was devastated but decided to try to have her build a relationship with our kids because I know it's what my husband would want.

Well MIL is dating again. She's been with her boyfriend for 2 months. He's met my kids twice. My kids stayed for the weekend at my MILs home and she had her boyfriend come over sunday. My two older kids, 14 and 12 got in trouble. My MIL kept telling them not to climb a tree and they did it anyway and a branch broke. They fell. Her boyfriend decided to punish MY children by hitting them both with a belt but my 14 year old son had to strip down naked for his punishment. Boyfriend actually tried to use that to justify what he did. He said he knows not to do that with my daughter because she's a girl and that would be inappropriate. I lost it when I found out. He left marks on both my children. My 12 year old daughters marks were on her arms but my son had marks on his back and butt because he had to strip for his punishment. When I went off he said back in his day you got spanked by your friends parents if you acted up in their house. I told him just because it happened "back in the day" doesn't make it right and he's not a friend. He's my MILs boyfriend that she's been with for 2 months and met my kids twice. My MIL said she kept telling them not to climb that tree. I did they got natural consequences from falling out of the tree. He had no right to spank my kids at all let alone with a belt and have one of my kids spanked NAKED! The reason I chose to make a police report is because he abused my children. IDC what anyone says. He abused them. And also with him leaving marks on their body I wanted what happened documented so CPS doesn't see it as I did it or allowed it to be done to them. I feel like my MIL will always pick men over anyone in her life and she needs a break from my childhood.