Just try really hard not to go straight to "My child is bullied"

Sometimes these things happen. And it's not because your child was a target or because the school doesn't take things seriously. Parents aren't generally told what consequences take place due to the fact that parents can go OTT. And they do this to stop that from happening. And to protect all children.

Talking to the school about being notified is important. But try to keep a lid on your reactions with your own child. It's very easy to go "WHAT HAPPENED!?" And then create fear and anxiety in your own child as a result.

It's awful when they get hurt at school and we aren't there to help them through it.
This.

It's really hard and I completely understand your worry. Ds1 has started this year too and there has been one or two things that have happened that have made me worried. He got scratches down his neck from a kid in his class for example.
I've tried not to make a big deal of it as I don't want to create anxiety for him. All I did was explain what he should do if the situation every happened again. I didn't say anything negative about the other child.
By all means you can say something to the teacher, but I would just say something along the lines of 'he's a little bit sensitive today, he got hit yesterday and was a little upset so keep an eye on him' that way your telling her that you're aware of what happened but not going in guns blazing. If it happens again then speak to her about what's being done about it.
What I try and remember is that these are only small children who are only just getting used to school. They are often overwhelmed and overtired and not quite sure how to handle situations. Some kids need a little extra help with impulse control.
Anyway, that's what I'd do, but you know your child and you need to do what feels right. I know it's upsetting