So sorry OP to read your post. Hope you’re ok, and your children are all ok too.
This won’t end well if your husband, who is meant to look after you, be there for you and care for you, is hitting you in any way, shape or form. A push here a slap there, then one day it’ll be something much worse. If he is a drinker too, that also fuels their temper and aggression.
It will be difficult, but for the sake of your own safety and for the safety of your children, report him. He should lay his hands on you whatsoever. This is physical abuse/assault, and will not be taken lightly.
The fact your FIL defended him when he’s been forceful with you before is a bad sign too.
It won’t be nice your children seeing this happen, but if you don’t act upon it now, they will suffer more in the long run. Seeing their mother emotionally, mentally and physically abused by their own father will scar them for life! Please find the strength to report him.
He sounds like he won’t even care if he had 50/50 custody of your DC anyway. If his family do get involved, stand your ground.
Speaking from experience it doesn’t get any better. Even if your husband apologises and said ‘he didn’t mean it’ or he tries to defend what he’s done, it doesn’t end there.
I hope you can find the courage to report him, fight for your children and be free of this man.
Keep safe and be brave.