schmee of all the wet arguments, that is the soggiest.
Of courses OP doesn't know what has happened in this woman's life. How can anyone? but she doesn't need to know to protect her son. What difference does it make? 
Nobody is saying OP should verbally abuse her, or start a hate campaign against her and her boy, but seriously, are you suggesting she doesn't mention it incase the woman has recently lost a child!?
Should I not speak to the postman when he yet again delivers my mail to the wrong house unless he is grieving for a relative too? Or should I not speak to anyone ever about anything?
Remember this woman is attending play group, wearing clothes and walking around. I think it's safe to assume her mental state is up to a conversation with another human who she might come into contact with at this group.
If something terrible HAD recently happened, a chat about her sons bad behaviour might - what - push her over the edge or something?
So OP should leave the group just incase?? Imagine she was telling her husband they do not go to the super duper exclusive playgroup anymore:
"Why not darling, I thought you were really looking forward to it"
"Well yes, but there is a 3 year old there who smacks the little one about"
"Well, ask him to stop"
"I have done, but he takes no notice"
"Well, that's not on, tell his mother, it's her responsibility"
"Good heavens, I couldn't possibly! I don't know anything about her. What if she has recently Had bad news? No, I'll just start taking him to the park instead"
Sorry but I'm not sure you are serious... This is the daftest notion I've ever come across!