Want to leave but can't because of the kids

My husband is a narcissistic abusive asshole. He is extremely emotionally abusive to me. He will say I'm worthless, I'm disgusting. He told me I wasn't pure. I had been raped by my foster dad and foster brother. My husband is the only one I've ever had consensual sex with and he continues to say I'm not pure. He will pressure me into sex. I've been wanting to leave for a long time, but I stay for my step children. Their mom is dead and I've been in their life for quite a while. The two girls are 8 and 10 and his oldest is his son who just turned 16. He is very abusive with them. He's more emotionally abusive with his daughters, but he gets physical with his son. On my step sons birthday his dad got drunk and got mad at him for no reason and started saying "You stupid piece of shit. You fucking retard!" He took my son by his hair and started hitting him and I am the only one that can get my husband to calm down and stop. If it wasn't for me he would continue to hit him. I'm the only one who can protect these kids. Ny son locked himself in his room and I went in and gave him a hug and let him cry. The only reason I stay is for these kids. I'm the step mom I have no actual rights to them. So I stay for them because they need someone to care about them. I just wish there was a way we could all leave this situation.