MrsW can you not see that to slap your dh was inexcusable? He was not threatening you personally, he was not hitting you, he was not even touching you.
He lost his temper so you think that justifies you losing yours and hitting him?
In front of the kids?
From what you've said about it all happening so quickly, I think you have a bit of a problem controlling your temper. I can't see how this has only happened once. Both you and your husband have tempers that you seemingly cannot control, so I really do doubt that this is the only confrontation your children have seen.
It might be the first time you have hit him and the first time he's laid a hand on you (and he pushed you away, he actually had more control than you because he didn't hit back, he just pushed you away from him) but I suspect it may not be the last unless you do something now.
You both need to admit that you lost control.
When he started throwing your things out of the window you could have then turned around and led your children away because to be perfectly honest, my first thoughts would be; "oh my God, my kids are going to be scared." I would have then told him that I was calling the police. I wouldn't have done, but the threat of that might have made him stop.
I would then insist he go to counselling.
You lost any high ground when you slapped him.
Is this a memory you want etched into your children's minds? Because it has done. You should both be shocked by what you did and very ashamed. This should be the thing that propels you both to take action but you will only achieve that if you admit fault. You don't seem to want to admit fault, you seem insistent that he started it and you were justified. With that attitude you will not move forward and your relationship is doomed.