TW: CSA please help

My 6 y/o M started his 4th day of school this week in first grade. I recently bought him a book about “body boundaries” because I could tell he started getting curious about topics and was just acting different. I know he’s getting older and I have taught him since he started daycare at 2 about the right anatomy of body parts and what’s right or wrong. Long story short. My husband and I just moved back to our hometown recently. We tried moving to the town he was working at previously because he’s a turbine tech and I wanted us to be together again as we recently have a 12 month old baby. While in this small town, we lived there for a total of 6 months before I said we needed to buy a house back home because I couldn’t do it anymore. The environment wasn’t healthy or safe. The 6 months we lived there I decided to go back to work. I work in healthcare at clinics, my schedule is 8am-5pm. My son was in kindergarten so he was at school 8am-2:30pm. Our at the time 6 month old was being baby sat by my husbands coworkers girlfriend who had a baby of her own and was helping to raise his step son. This woman was a substitute teacher, we met during my sons open house. Well after giving my 6 year old and 1 year old a bath tonight to get ready for school. Out of nowhere, during bed time he said “I have to tell you something (the kid) did but I don’t want to” Me: please tell me, you won’t get in trouble My son: “ he was doing inappropriate things in front of me” Me: you can tell mommy. You’re not in trouble. It’s good you’re telling me” at this point I wanted to fall apart I knew what was coming. He proceeds to tell me this kid who’s a couple years older put his penis on my son and made my son put his penis on him and laid on top of him. It happened in the backyard. My son said it happened “ten times” mind you this kid doesn’t live with dad and step mom primarily. He’s one week on, one week off. I immediately tell my son he can lay in my room tonight and watch tv that I’ll be right back. So I go in my car to ask my neighbor to please come over to get my baby for a bit because I need to call my husband and I tell her what happened. It’s 8pm. She runs over to get my baby and I immediately call my husband and I can’t even get the words out. I tell him what happened and I let him know I’m in the car so our son doesn’t see me like this….anyways my husband calls the other parent to let him know what happened and what we should do. The dad completely denies it and starts accusing our at the time 5 year old. My husband Calls me to update me and I didn’t want him to let the other parent know but it’s too late. While otp with my husband the other parent calls. We’re on three way now. His baby mama of the son is on the call and is super aggressive, calls me a liar, the other parent threatened us because he said “you know me and your husband work together right? You’re trying to ruin his life” then the mom calls me chicken shit and says if I really cared about my kid I would’ve handled business there instead of leaving, but I didn’t know any of this UNTIL tonight. We just moved here end of June. They go on to make more threats, accusations, say they’re gonna get authorities involved, they said we need a lawyer, they started saying my husband is a bad dad and I’m a bad mom because I worked long hours…and etc. I DONT KNOW WHAT TO DO. I don’t want to traumatize my son. I don’t want him to feel like he should’ve never told me. And I tried to tell the parents what my son told me and they didn’t even want to hear it. I didn’t get a single sentence out. They cared more about denying it rather than even trying to know what the accusations are. They even said their son doesn’t know what any of that means. Etc etc.