Most North American cities sadly don’t operate at the level of trust required for a system like that to work, as much as I agree it would be better for everyone.
IMHO, if you have fare gates they need to be tied into a parent control system so that parents to limit where their kids are allowed to go alone. I've never seen the implemented and the details are important to get right.
I didn't use it that much, but did see ticket checks on one of the trips, nobody was without one.