@hawkeye10,
I'm agreeing with several here (oh, ebrown, hawkeye, and boomerang, and probably more, re rules). I don't like, looking back now over decades, the whole thing about strangers. The italian word for foreigners is stranieri.. not that foreigners, strangers shouldn't be words, but those can bring up fear of people, and xenophobia keeps living around us.
I'm for rules to start with, as the children haven't the experiences to have refined judgement re who is who, with increasing information to inform their choices as they grow.
I smile at children in the grocery store, not all the time, but quite a lot, while I also try to make the smile to the parent. I have a frisson once in a while that someone might think that's creepy. Just recently I smiled at a young woman trying with difficulty to install her child in the grocery store cart, and she stopped and looked me and smiled back. I'm an older woman, so that might have been an it's ok signal.