For the same reason anybody takes a photo in similar situ.
It is not like they get pissed, or shepherd their kids away. They see me. They know i am not just taking photos of random kids
You r asking the wrong question anyway...........why does nobody ever feel a need to confront me when i shoot their kids.?
Answer........because it is OBVIOUS i am just a photographer, with 2 cameras hanging from my neck, shooting in public spaces with all kinds of normal activity taking place in a crowded city. It is normal.
Whether or not they know the definition of, or have ever heard of "Street Photography" is completely meaningless.