This relationship sounds like it's teetering on the edge of a cliff.
1. Why isn't your wife willing to listen to you at all? When you talk do you go to the arguing place like right away or do you sit down and tell her what you need/want?
2. Something that's missing that would be a big issue for me: You say your wife is out every weekend and you spend that time taking care of the kids. Where is the time for you and her to be together and when do you get time to yourself?
3. Why is the dom/sub lifestyle so important to her? You know what you don't like about certain aspects of it, but have you guys ever discussed why she feels like she needs it?
4. If her dom knows she has children (and I assume he does) how does he think it's ok to order her to come over there every weekend? I know that both he and she enjoy that situation, but as you stated, the children are suffering from not seeing their mother. I know a lot of people who are doms, but there are boundaries of sorts and there is enough respect for their subs there that the life outside of that means something too. As a sub, she may be the kind of person compelled to do everything her dom asks of her. As a dom, it sounds like he may be asking too much. Have you tried talking to him instead of her, or is there no way for you to have a calm conversation with him? Normally I don't recommend going around your partner to try to resolve the situation, but I have a kid and I have a weak spot where kids are involved and if talking to him would help- great.
It sounds like if you want to save your marriage you may both need to be open to and agree to therapy- to work on the communication if nothing else. I know adding another person (even a counselor) can't always be the answer, but in this case if your communication with each other is that poor she may need to understand why you don't want to have bruises on her face (among other things).