Yes, I've been in a similar situation. I adopted my daughter when she was 6 and she was full of anger and did not want to be my daughter. I was her 5th foster placement in 1 year. She did warm up to my boyfriend. I think in part it was because he was a warm caring person who did not get upset when she acted out. My boyfriend and I did not live together and only saw each other a few times a week. When he was with us, my daughter was his focus. He talked with her about how she felt. He listened to her a lot. We went on walks, to the park, to the mall together which was a good time for him to show his love. Frequently he carried her on his shoulders.
Your situation is different in that your children have always had you and now it feels like you love your boyfriend more than them. I know you don't but that is how they feel.
They have good reason to be frightened. This is the 4th man who might walk out just like the other 3 did. In your daughter's mind you have not protected her from this pain.
No doubt their anger and your frustration with your daughter's hitting and kicking has resulted in you being angry. I suspect that the two of you share angry words before the hitting starts. It was that way with me. Yes, my daughter also hit and kicked me.
We were in counseling and I strongly recommend that you and your daughter start with counseling. It sounds like the anger has taken over your lives. If you don't have insurance that will cover it there are clinics that charge on a sliding scale. If you go to church your pastor is a good person to go to.
Here are some suggestions that you could try.
Find a way to control your own anger. If you are angry don't talk. Never argue.
Spend time alone with your children without having your boyfriend present. He could watch TV if he lives with you. Bedtime routine is good for this. Take 30 minutes at least to be with your kids while they are winding down for bed. Play a quiet game or help them with getting into their jammies. My grandaughter who is 7 still likes to have me put her jammies on when I'm there. Tuck them in, read a story. Hug them and in a quiet loving voice say "goodnight. I love you." Make this time pleasant. It's a time for showing love.
This will be difficult at first. If this is a new routine it will take awhile for them to trust you to continue doing it. And your daughter will probably be angry with you. Just keep working on it.
Find a way to spend time alone with the kids every week doing what they are interested in doing.
Make a plan with your daughter for how both of you can better control your anger. Tell her how you feel and listen, really listen to how she feels and accept those feelings, even the angry ones. Tell her it is OK to be angry but not to hit and kick.
When my grandchildren get wound up in a certain way I know that anger will be next. I send them to their rooms to calm down. It's not punishment! It's a tool to use to learn how to control your emotions. Tell them this. Sometimes when you're angry send yourself to your room. Time in the room is private. No one comes in unless they're invited and no one comes out until they're calm.
You and your daughter could work together to find ways that help her to calm down. Perhaps she likes music or writing. My daughter wrote a beautiful poem when she was 10 or 11 about how anger comes barreling down the track at her like a train. Perhaps you could help your daughter make a cozy place for herself in her room. She can go there anytime and it's available for cool downs. Give her something special from you to her that shows love. You could even shop together for it. A necklace, a stuffed animal, a doll, etc. Whatever she is interested in.
Perhaps she can have a large pillow to hit and kick in her room. A stack of newspapers and/or magazines to rip to shreds. If it's safe she could run around the block. Perhaps the two of you could run around the block. She could do jumping jacks. You could turn it into a humorous break from the anger by suddenly starting to jump yourself.
Humor is healing and usually stops a tirade if done at the right time. One parenting class I took said to do something silly that your child wouldn't expect you to do before you both get angry. Be sure that your humor is not mocking her.
Is it possible that you and your boyfriend are trying too hard to get her on your side? And/or that he's taken over some of the discipline? I've learned from several sources that a step-father should stay out of the discipline. Have you focused more on getting the children's acceptance than on how they're feeling about this intrusion?
Try to talk with the about how they feel. Share your feelings with them too. Acknowledge that they have the right to feel anger and whatever emotion that they express. Be compassionate, understanding that this is a frightening move for them.
I also recommend that you tell your daughter that you made a mistake and that you will help her with a Halloween costume. Taking away the costume is too big of a consequence even tho hitting is really serious. Taking it away only increases her anger. It makes her different from her classmates which increases her feelings of worthlessness. Discipline needs to be immediate ie: take away something in the present like turning off the TV. And the discipline is best if it's related to the offense. Sending her to her room to cool off for example. But send her to her room in a firm but also kind way. Talk about what will happen ahead of time, as I suggested earlier, before there is a tantrum.
All of you are in a very stressful place. Changing the anger to acceptance is difficult but you can do it, especially if you get some help. The most important ingredient is compassion for what the other person is feeling. We teach compassion to our kids by modeling it as well as by talking about and accepting their feelings. There are not bad feelings, only bad ways of expressing them.
I wish you well. Changing this will take a lot of time; perhaps months. You'll need patience. You'll need ways to get away and take care of yourself.