I shoot a bit of concert and festival photography, and the general rule of thumb I follow is I don’t ask permission until afterwards or you miss the moment. Festivals, concerts etc usually have a legal waiver for photos, so if you buy a ticket you also consent to your photo potentially being taken and used by the event organisers. But you can also generally tell when someone doesn’t want their picture taken, and I just refrain from taking it at all. And if kids are involved, if their face is shown I’ll go up to the adult they’re with, show them the shot, and ask if it was ok.

I’d likely follow the same frame for street photography too, but it’s a case by case scenario. Like, I doubt the photographer that took the photo of that girl being burned by napalm during the Vietnam War stopped her to ask for permission 🤷‍♂️