I've been a nanny for 38 years and I don't tolerate any foolishness from children or parents. Knock on wood, day one on the job I will sit the children down and tell them not to do anything to me that they wouldn't like me to do back to them.
I told them if they hit, kick, spit or whatever to me I will do it right back to them. And they told they parents what I said, which they should, and so far it worked well for me.
I could go to work and pull off my watch and ring and put them on the table and no one will touch them. I knew of many nannies who got hit, kick, spat on, and called names. And their purses and watches were damaged by the children. And the children's parents never reimbursed them for the damages.
You see nannies with children believe that they have to accept garbage from peopled and their rotten children because they have bills to pay. But that shouldn't be the case, no one should have to work in a horror home.
Say, had you hit him back his parents would have called the police on you, you should have called the police when he/she hits you.
You need to leave this job, and the next time he hits you call the police and his/her parents and let them know that you are leaving the job. Please don't stand for this treatment.