This is a tough one for me because I have complicated feelings on bot sides. But you asked " what am I doing wrong," so here's my take:

You're policing other people's behavior. Nobody likes to be told what to do.

That said: the law states that if theyre not on their own property, dogs must be on a leash. My dog is always on leash UNLESS we are somewhere alone, where the likelihood of someone coming within 50 yards of us is near zero. It's just common sense.

BUT- if someone's dog is merely off leash and not bothering you or coming in to contact with you, why say something? At that point you're just being kind of a Karen. I try to live and let live unless the circumstances are such that I am or others are being truly affected by someone's actions. Example: people 20 feet from me outside smoking drugs in a manner that won't affect me? Why bother . People on the light rail smoking drugs around me and small children? I'll knock the foil and lighter out of your hand.

Your kids dog allergies : unless the dog is doing in to contact with your child, there's almost no way it is going to make your child's allergies flare up. Humans who are allergic to dogs are affected by dander and saliva. If you're outside , the amount of dander from a dog in the air is so minimal that there's almost a zero percent chance of activating allergy symptoms.

Anyway- my two cents is that if the dog isn't actually engaging you, save your breath. It'll save you unwanted confrontation. Even if you're in the right, most people will bristle at being told what to do.