Assuming she actually leaves him, there will be 2 separate legal cases: the one about his criminal charges and the one about their divorce, which will include custody.
As others have said, if you are worried about your safety, talk to the prosecutor first to see if you can be of use. If they think your testimony is useful, they will tell you what they need. It is often an interview where they ask you a bunch of questions and take notes or even record it, followed by being asked to appear in court to testify.
If you are in touch with the wife, you can let her know you will be available to provide your testimony if she files for divorce and needs your input for custody, but only if her lawyer thinks it’s necessary. In this kind of situation, it tends to involve starting with writing an affidavit. From there, you might be questioned at trial or in a separate procedure regarding evidence. Your ex might also decide not to question you. In that case, the judge would have access to your written document and the parties can argue about whether it’s relevant or not.
It’s also possible that a youth protection file has been opened. It’s pretty common when someone is charged with assaulting a child. In that case, information about his history could be used informally by the case worker to help them decide whether they need to pursue measures to protect the kids, or it could become testimony in a hearing about the kids’ safety and measures that need to be taken to keep them safe.
Others have touched on the risks for yourself and your daughter, but that’s more of a social question than a legal one, unfortunately.
If you haven’t personally witnessed his violence against her or her kid, you can’t testify about it. You may be helpful as a collateral witness though by describing the violence that he perpetrated against you and your child. Talk to the prosecutor and see if they are interested in your testimony.
Do not speak to his wife! She may leak this to your ex / her husband. Contact the prosecutor office and speak to them. If they need you, they will contact you.
Zara said:
Do not speak to his wife! She may leak this to your ex / her husband. Contact the prosecutor office and speak to them. If they need you, they will contact you.
But it seems as if OP’s daughter is involved with the current wife since she has contact with her half-sisters. So, I’m assuming the OP has contact with the wife to facilitate these interactions.
Zen said:
Contact the wife. Tell her you will testify to his character. Tell her how you got away with your daughter.
This is horrible advice. Do not follow it.
Why? Why is doing the right thing and helping a fellow woman and children get away from an abusive POS not good advice? I hate society. Full of cowards and enablers.
@Zen
Some of the other comments explain this very well. You might want to look at them. If she contacts the spouse, she could end up being targeted, but if she contacts the prosecutor, they advise whether her health would be warranted. Do not contact the spouse.
Speak to the wife. So many times the victim and victimizer get right back together and then you’re the target. Sometimes it’s better to stay out of it and let her, the DA, and the courts figure it out.
@Rafe
I see this daily. Scum like her ex are in jail, and the victim calls, messages, or FaceTimes (with their still busted-up face) the piece of garbage, apologizing for him being there because it’s all her fault. She should have never called the police. I’m sorry, baby. Please forgive me. I’ll just ignore my subpoena and not show up for court and then you can come home with me… blah blah blah . OR… the trash tells her it’s all her fault and advises her not to go to court or to tell them she was lying. . Also, talking to the DA is risky because he may mention her willingness to help, and then the scum finds out anyway. I’ve seen that happen also.
Pace said: @Rafe
That’s exactly why OP shouldn’t speak to the wife. She should speak to the DA/ADA handling the case.
This is what I’m doing! I told her friend that I am willing to help her if she ever needed it and that he WILL end up hurting her so the friend knows the moment she decides to not go back to him, the friend can reach out to me. I hope she does!
. OR… the trash tells her it’s all her fault and advises her not to go to court or to tell them she was lying. 
. Also, talking to the DA is risky because he may mention her willingness to help, and then the scum finds out anyway. I’ve seen that happen also.