Best to go, just to show a bit of support.
Be ready to call the police, if it does look like getting nasty.
This sounds like a bad one. it’s the kids that have to be thought of first, and what’s in their best interests. Is it possible to move them to a neutral venue, and let the adults try to sort things out? The sight of parents rowing/fighting is very traumatic for children, and can have long-lasting consequences. Best to keep any upheaval to a minimum, for them.
If his parents are likely to cause extra trouble, then make them stay outside/tell them to **** off. It’s not really their business anyway; it’s between the two parents. They are likely to make things worse.
Go with your missus, and be prepared to explain you are there to support the woman, and ensure no harm comes to her. Be a man; no-one wants to be in this position, but you’re well out of it, as far as the impact goes. It’s not you that’s affected. Go and help a fellow human being.
If it kicks off, I can guarantee you police and Social Services will be involved. maybe they should be.
Good luck. Hope it ends as peacefully as possible.