|I hope that the OP's reaction gave that mother pause and led her to have a discussion with her son about respecting others and not invading their space.
Not to sound ornery but frankly if it had been my son who rushed up to some child yelling 'friend, friend" and gave the child a big hug and then dashed off I really don't think I would interpret the mother looking at me and frowning as:
"Gee- maybe her child has immune issues"
"She seems upset. Could she being thinking about the cold and flu season and the possible germs my child could have just spread?
"Maybe that child has personal space issues"
"Guess it times to talk to my son about respecting other and the importance of personal space"
A frown like that could be interpreted a thousand different ways
“Guess they are out of stock on xyz, again”
“Someone needs some coffee
“Great, another up tight mom”
Chances are I would chalk it up to another 'whatever" moment, if the frown even registered with me at all. Bottom line is complete strangers are complete strangers. They have little to no impact on anyone’s daily life. By the time I caught up with my son, finished shopping and got to my car I don't think I would even remember the moment. And chances are if I did I would remember what my son did, not what the mother did. Frankly, a stranger does not have that kind of impact on my life.
Do you (general you, not directed at any one poster) think any one is really going stop what they are doing and try to teach a 4 yo a life lesson while said child is dashing around the stores? And if they have not been teaching their children respect, manners etc all a long I doubt one women in a Walmart, shaking their head and frowning, is going to get them to go home and start, KWIM?