Kids need to bring stuff. That could be hockey equipment, a drum set, or a clean playpen.
Kids wander off if not contained. A kid may get off a subway at the wrong stop, lost in the crowd, along as the rest of the family continues on down the line. A kid may fall onto the tracks.
Kids should be kept from licking public spaces. Kids should be kept from eating food found on the ground. Kids should not be able to play with hypodermic needles.