Heart hurts

My heart hurts for my children. Today my youngest and I were playing and out of no where she asks me if [enter name] who is my bf, well asks if can be her stepdad. I'm silent, scrambling to come up with an answer. So I turn it around and ask her why she asked me that. She told me she would like him to be her stepdad. I ask why exactly? She responded with "because my real dad is mean to me." I just hugged her and told her I loved her an that I was so sorry that he is mean. I didn't want to grill her and get specifics because that would upset her. But I went over how certai things are bad and how no one is allowed to do them to her, how she can tell me or a teacher no matter what the person who hurts her says.
Every time when the day comes for them to go to their dads for visitation I feel like a monster for making them go. They tell me they don't want to go. But I can't keep them from him. Why is it that the law isn't letting me keep my kids safe? They make me send them to a man who was accused of molesting two girls years ago. A man who steals identities, a man who drives with no license, a man who doesn't use car seats for my kids, a man who physically and emotionally abused me for years, a man who owes a ton of money in child support for another child he has, a man who wouldn't pay support if I went for it for my kids, a man who is so brutally mentally unstable that I truly fear one day he will kill someone.
I wish I could take my kids and hide us. I don't want them exposed to his abuse mentally and or physically ever again.