Firstly, this truly was an accident. We are both very against physical punishments. He was extremely apologetic to DD when it happened and he feels extremely guilty. She is okay, just a bit shaken and says it stings.
DD(7) is autistic and was having a very big meltdown. During these she can become violent and she was trying to hit us with anything she could find. DH went to slap one of the objects away and accidently caught her instead, hard enough to leave the raised imprint of 3 of his fingers.
We want to get ahead of this by telling the teacher about the incident as it'll likely be seen in PE. But will it be reported? They are fully aware of DD's meltdowns and are trying to support as much as they can but just looking at the mark from an outside perspective I'd probably think the worst.