So he deliberately threw something right next to the baby's head?
I thought when you said that was an accident, that you meant that it genuinely was - as in, he had no idea that the baby as nearby etc.
You must not allow him to come back into the house.
If his anger is uncontrolled enough to allow him to make what he KNOWS is a violent gesture towards the baby, you cannot let this resume. You cannot.
That action was deliberate, you know - designed to frighten you, designed to let you know that if you don't appease him, he could do something terrible. Calculated so that he would narrowly miss the baby, just to put the fear of God into you. Or maybe just to punish you. Whatever.
He might have miscalculated and not missed.
He might, next time, be so overtaken by anger that he doesn't calculate at all.
He might not even care next time.
The important thing is, he is a danger to the children.
Please call the police and get tonight logged. For two main reasons:
-you do not know how any of this is going to pan out, you may find that you need a record of this;
-you will come under pressure to let this go, informing the police is one extra level of protection against this being swept under the carpet. Even if all it does right now is give MIL the metaphorical shake she needs to get her to butt the fuck out of it and realise that what her darling boy has done is indefensible.
I would not meet them tomorrow. Lock the doors, call MIL tomorrow am and be coldly non-negotiable about the whole thing - you have rethought, you will be spending the day with your children as they have had a bad night/nightmares, they witnessed all their father's violence and you will be spending the day with them. You have nothing to say to your H at the moment and think it best that he stays away for the foreseeable future.
If she has the absolute fucking brass neck to argue, cut her off with a sharp reminder that her son is a violent aggressive beast and if you hear another bleat in his defence from her, she can kiss goodbye to any respect you have for her, and probably kiss goodbye to a relationship with her grandchildren at the same time. Including the baby that her son threw an object towards the head of. Just incase she wasn't aware of that little delight.