I'm going to go in a bit of a different direction here, but let me explain.
1. Yes, your GC are sadly being abused and you need to do something about that.
2. You want to help your daughter. I get that.
BUT.
You can't go in with guns blazing and take her out of there. Let me explain.
Women in abusive relationships are often severely isolated and beaten down. Often their self-esteem is in the toilet. They do not believe they could make it on their own. Their partner has taught them to rely on him for EVERYTHING.
Secondly, even if she does leave, there are risks involved. She can't just take the children with her - she could be accused of parental abduction. Moreover, there is a potential risk that the abuse could escalate and even become homicidal if she tries to leave. This isn't alarmist thinking. It is a documented fact. There are people who have been killed by their abusive partners when they have tried to leave. That leaves your GC with a dead mother and a father in prison. Their home would be essentially destroyed.
What you CAN do is report the child abuse. You can also suggest that your daughter look for a way to contact a shelter or a domestic violence hotline. If she is working away from home she may be more comfortable to contact them from her work or the local library, in case her partner is monitoring her phone and internet use. If they have a modern understanding of domestic abuse they won't just counsel her to LTB; they will also speak to her about safety and help her make a decision that will work for her.
Yes, ideally she would leave him. I agree. But the reality is that it's not that simple. The most you can do for now is support her and contact alternative agencies as the others have suggested. You might also see if you can help her put together a go bag - clothing, important documents, and cash. (Cash is best for a quick getaway because it can't be traced and can't be frozen.)