Imagine telling somebody about what he did.
What is your reaction to the idea ?
If it is fear, that he might hurt you, or punish you in non physical ways then potentially on some level you are probably not convinced that this was a totally out of character "one off". In which case decisive action to get yourself away from him is an immediate necessity.
And yes I would tell somebody, if he can't be honest about the enormity of what he has done if the event is discussed between just the two of you then he needs his behavior reflected in the horrified faces of people who know you both, and care very much about YOUR well being in order to see it for what it is.
My one single piece of advice is do not let this become "our secret".
If it was a one off then he needs to carry it around for what it is, a godawful, outrageous and damn near unforgivable thing to do, rather than something justifiable, half forgotten and detached from his self image.
When you chose to hit, or let yourself get out of control enough to hit, then you have to live with the natural consequences of that. Being known as somebody who does not belong to the group of people that others can declare "oh he would NEVER raise a hand".
If he is mortified, then so be it. It may be the emotionally response that saves him from ever repeating such an action again.
If he is not, if he seeks to justify and seek out those who will justify on his behalf and make his action YOUR responsibility (eg "she drove me to it"), then run for the hills, because the possibility of round two is far to high to risk for you, let alone your kids.
Hand on heart I believe that had she not kept his secret the sight of my mother cowering in anticipation of a slap across the face would not form some of the most vivid memories from my childhood. One such "film" in my head is my earliest memory. I would have been around 18-24 months old at the time.
It was occasional rather than regular. Infrequent (sometimes years apart) rather than constant, a slap rather than beatings. And yet it totally coloured my vision of what a relationship looks like for a long time.
Both of them colluding in keeping his secret meant we were forced into being guardians of his secret too.
Which is really not fair on a child.
I absolutely appreciate how hard it is for a spouse to whistle blow. So much fall out to consider. However the alternative to whistle blowing is to leave the children to either actively avoid speaking out, playing their in an unspoken family pact, or to screw themselves up into doing what needs to be done by telling somebody because the adults can't or won't. The sense of shame is not felt exclusively by the primary players, the audience carries it too if allowed to become the dirty laundry that must not be washed in public.
And a big, fat hug, because you must be reeling and in utter shock right now. I sincerely hope you have somebody in your life who will listen and take a stance that does not allow this to be minimized and swept under the carpet.
You did not deserve this, no matter what you said you are not responsible for somebody striking you (unless you have created a devise that allows you to take control of another person's central nervous system), you are entitled to a groveling, unreserved, non blame shifting apology at the very least.
And your children deserve, need, depend on, their mother believing the above.
I am so very sorry this happened to you. I am so very sorry your DP let himself down like this and I hope for everybody's sake it was an out of character act that comes to be deeply regretted and never repeated.