I neither want nor need to be a taxi driver for my kids. From the age of seven they have been perfectly capable of getting to school, orchestra practice and sports by themselves using the subway, local trains and buses. The drum kits and equipment can be stored on location, and whatever they need to carry (the cello is heavy) builds strength or can be wheeled.
At their age, back in the day of no smartphones, if I got lost in the city I could ask any adult for help, and they would be delighted to help me.
At one time, mine were these ages: 8, 6, 4, 2, 2, 0
Doing that on the subway would be nuts.
I actually know somebody who, as a kid, got off the subway at the wrong stop. That really happens. The crowd seems to be leaving, the kid goes, and before the parents can react the subway is moving again.
Activities need not revolve around consumerism. Find a less-densely populated area, and there will be wild space like forests and streams. Backyards can be pretty big too. Consumerism is more of a city thing, since there is no backyard and no forest.
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.