OP, I am sorry this happened at your child's school and I can imagine it is very worrying and upsetting for you. Especially with not knowing who the child was, that must weigh on every parents mind.
It worries me that your child was so upset though, a child sobbing about this at that age would ring weaning bells for me. Their reaction is not what I would expect. Which reinforces why it's so important that the school have discussed this, you admit yourself to filtering what your child sees and knows, so that is why it's so vital that schools so take the stance of informing children about what is appropriate or not.
Your child's extreme reaction could just have been because they have been sheltered and not realised people have ulterior motives, but I think an open conversion or why your child is upset needs to happen. And if it's just because of what the teacher said, then it's impotent that your child knows how to react in those circumstances.
I do think the school should have followed through with parents though, hopefully they did by Friday. I think in this case it would have been good practice to inform all parents what the children were informed, what happened and link to the pants campaign so parents can reinforce a united front of how to protect their children.
I know you have stated you haven't discussed things like this with your child, I think the shock that these things happen has really upset them and I think from that you need to play a more active role in teaching your child.
Our of curiosity what would you have said to your child? If thing would have gone the the way you wanted how would you have informed them or would you have?