I am so very sorry for your situation.
I was a child who grew up with an alcoholic step-dad (and a little sister who was his biological daughter). I also went on to stay in an 11 year mentally / verbally / emotionally abusive relationship so I can connect to what you are saying on different levels.
My step-dad (my mom did eventually leave him when I was still in elementary school) was not abusive to my mom (other than being neglectful, unreliable and drunk all of the time as far as I could tell), but was very abusive to me. He definitely made a point of showing that I was the step-daughter and my sister was the perfect apple of his eye. My sister was bought presents and could do no wrong as a baby while I was locked in a room for hours (my mother went to night school and worked during the day, my step-father would stay at home and 'watch' us which, in his mind was locking me in a room and watching hockey). I was smacked around, had my neck roughly grabbed / squeezed, had my hair pulled and was just generally in fear of him because of the aggression he showed towards me. No broken bones or anything major but man the emotional trauma lasted for YEARS after my mom left him and as a teenager / young adult I still held onto resentment and anger that I only now understand after years of reflection.
I went onto to dating a boy in high school when I was 15 and latching onto a deeply unhealthy co-dependant relationship. His mother was also an alcoholic and he carried a lot of baggage from that (as I did from my past). We both brought lessons from our parents into our relationship... he had learned how to be an expert liar, sneak and cheat (which is how his mom got by in her various relationships in which she took advantage of men) and I was good at looking the other way, making excuses for his behavior, forgiving him (as my mom had done for her husband), holding out hope that he loved me, and thinking that if i just held on long enough it would just get better somehow. He also developed alcoholic tendencies and addictions to some other drugs. We got married at 22 and at 26 he was heavily pressuring me (while using mentally abusive tactics) to become pregnant. He argued that getting pregnant would solve all of our problems (he would stop cheating, drinking and being deceptive if he had a kid to motivate him to turn his life around he argued). Something snapped inside of me. I knew I wanted to be a mother... very very much so. BUT there was no way in hell I could EVER bring a child into this situation, all I could remember was the torment I went through growing up in a house with unhappy parents and an alcoholic father figure. I realized that I had to either face never being a mother and staying with this guy or leave. So I left.
Left EVERYTHING I owned except for my car, my clothes and my laptop and moved into my mothers house again. I went through major depression, feeling like I was a burden to my family and had nothing to show for the last decade of my life. But I was also really excited for the potential to come and the relief I had given myself of not relying on this man anymore for love or affection or security. I faced a year of harassment from my ex, as he tried every tactic to reunite with me... from promises, to bullying, to downright stalking. I had to pay for the entire divorce by myself because he refused to cooperate and to spite me, he destroyed most of my possesions in the process I was hoping to recover (including items from my dead father that had deeply sentimental value). Yes I could have taken him to court and attempted to get some of the cost back but lawyers are expensive, it takes time and I was just so done. I just wanted out. I rather would have walked away with a clean slate then dragged it on for months / years. It was what was best for me and my health at the time. I also had no money to put towards a lawyer (I had been laid off of my job the same month I left him... and was broke in every sense of the word).
I am now 29, 8 months pregnant and married to THE MOST loving, caring, supportive husband I could ever ask for. I spent a year by myself and spent that time in deep reflection, and getting to know myself and my patterns that got me into such a mess in the first place. I made a promise to myself not to date anyone and just focus on me, what I needed, and what I needed to do to never fall into that position again. I moved out of the country to get a job that allowed me a lot of financial freedom, paid off all of my debts, lived in my very own apartment by myself for the first time in my life.
Why am i telling you all of this?
Because I want you to know that ...
a) You and your children in your current situation are suffering. I know exactly what it is like to be in both your son and your situation and it's NOT WORTH IT. No one in this world is worth making you feel anything less than a valued, loved, important human being. If either you or your son are feeling less than that, it's time to change something.
b) It will get better. It may seem hard right now and downright impossible... but I promise you IT WILL GET BETTER if you leave. I never realized what kind of hell I was living in until I left it. The relief, the confidence, the peacefulness... I hadn't realized what I was missing out on until I had it again.
c) If I can leave everything I ever owned, my pets, my financial stability, and my country, all while penniless, you can do this. You have a family... you are their daughter / sister / aunt / etc. You have their grandchildren / nieces / nephews / cousins , etc. Let them rally around and support you. Don't be afraid to ask for help. There are many programs out there to help women in your position out there. Don't be afraid to use your family and your community to get out and in return, you will be able to turn around and in time, transform into an independent, productive, loving, caring, happy person who is able to provide her children with a happy, healthy, home.
d) Your children need a happy healthy mother. You can't look after them properly if you aren't looking after yourself and your emotional, mental, and physical well being. He doesn't have to be your husband to be your daughter's father. After my mother left my step-father he still was a constant in my sister's life (for awhile at least... that's a whole other heartbreaking story there). Your children will learn how to treat people, and how to expect to be treated by following their mother's example. Show them to expect nothing less of kindness, respect, compassion and caring from a partner and to not accept anything less from themselves when interacting with other people. You don't want either your son, your daughter or your expected baby to learn that this is what a relationship should look like.
e) Yes he might offer to change... I know my ex did the 1000 times I threatened to leave him. Many promises, many half-fast attempts that quickly fizzled out. Maybe your partner is different, maybe he actually might change for the better of his family. Maybe. But that's work for him to do on his own time and in the meantime, you need to look after you, and your kids. If he puts honest effort, time and work into correcting his problems (including facing his alcoholism and not drinking anymore) then explore the option of being a family and rebuilding that trust again but only then. My experience with alcoholics and drug abusers is that they won't let it go until they've hit rock bottom. Maintaining the family at your and your children's sacrifice allows him to maintain his comfort level and enables him to keep doing what he's doing... while everyone else suffers for it.
I'm really sorry you are experiencing this. I see a lot of myself in your situation. It can and will get better. I promise you this. You are worth it, your children are worth it, and don't let him or anyone else try to suggest otherwise.