I agree and disagree Offred. Without wanting to be too academic about it when OP needs support.
What I was trying to say (and didnt very well) was that if Blackberry is in a position where her DH is in the dominant role in her marriage, if he is also 'damaged' (in the garlic sense above) then her role is to be the victim. So she will naturally seek power-over somewhere else....and kids are the perfect targets.
He needs to look good in the eyes of the world, he doesnt feel good inside, so he seeks to feel better than her by belittling her.
She doesnt feel good inside, she is in a stressful position (as would anyone who is dealing with bouncing 7yo), she seeks support (from the one person who should, the one person who cannot give any of his power to her) and when it is not forthcoming, loses it with the child, although the correct target should be DH. She has seen what happens when she did stand up to him, so the child is the logical person to act out on.
And probably at some level, OP, you felt cowed by him before now.
I doubt this was the first time that he has threatened you, even if it was a first hit.
DH wont/cant give her support...that would make her his equal or his better....which is not on as far as he is concerned.
Blackberry I hope this does not come across as patronising to you.
I have been where you are with a very domineering and abusive partner.
You may not even see him in those terms, I didnt.
I am ashamed to say that there was a lot of shouting at my DC which stopped very shortly after we split.