S.H.
Ann probably knows how her son is.
At in school, the Teacher probably has told Ann of the problems her son causes etc.
So, Ann probably knows... all of this.
We are not a fly on their wall at their home. We don't know how Ann handles him or not, and we are not her family or the boy's parent.
Now, speaking for myself personally: since my kids were about 2 years old... I TAUGHT them, how to speak up, how to handle icky kids, how to DISCERN social interactions etc. So that... when crappy things like this happens, even when I am not with them.... THEN THEY will know how... to handle icky situations or how to handle icky kids.
THAT is another way.... to "problem solve" icky social situations for kids.
THEY too, need to be taught, HOW to handle, things, too. Because, reality is, we are not always with our kids in their midst... and so we as a Mom cannot intervene or navigate or referee, every little social conundrum they have. And I also teach my kids, to tell me if anything icky happens, and/or to tell the Teacher or any supervising adult, who is there. And, they do.
So, that is what I do with my kids.
So beyond just wondering about if you should tell the Mom of an icky kid, that their kid is icky... you ALSO talk with your child, and teach them about how to handle things too, if you are not there with them. It is not about just expecting a child to "handle" EVERY social problem by themselves... but that, IF they are in an icky social situation or being treated not nicely and you are NOT there beside them... then THEY will know how... to navigate themselves, against icky, kids.
For you, you can tell Ann, or not.
Or, you can correct her son when he is with your son... and tell him. When other kids are playing with my kids and they do something untoward... *I* do correct... them. And if I know the parent well... I tell the parent and ALSO about how I corrected the situation, with the kids, including her's and mine.
Per that boy hurting/pinching your son at the play-date... I would have, RIGHT there, corrected that boy. AND made him apologize. And I would have told, the Mom. AND I would have, RIGHT there... told my child to speak up, too. And my kids, do.
And if it did not improve, I would have, ended the play date.... and went home. Or not have ANYMORE play dates, with them.
ALSO, per my own kids, I have always taught them... HOW TO CHOOSE... friends, and what a friend, is.
IF my own kids were mean and icky to others... I would want to know.
But again.. if a child is this icky and mean to others CONSISTENTLY... then I would think, that that child's school and Teacher, would ALSO be having problems with that kid, in school. So the Mom, MUST know. But she is in denial. Or she is embarrassed about her son's behavior. Thus, she acts all "shocked" when her son is like this.
Just because that boy is the "youngest" in the class, it does not mean that all "youngest" kids, in a class, act this way.
Both my kids are late born and one of the youngest in their class, but they are not like this.
Age of a child, has nothing to do with the child's untoward behavior.
Why don't you just stop having play dates with that boy?
And you TEACH your son... about how to choose friends, and how NOT to be treated badly by others.