Currently, children must pay $1.10+ per bus ride (depending on age). If we let them ride free, then they will get used to using the bus system at an early age. Plus, 16 year olds then may be less excited to drive a car, etc, if it's cheaper to bus than drive. And parents might be less willing to drive their brood around to their after school activities and social events, if they can just say 'take the bus, kid'.

I, myself, was hooked when I received free bus rides from my school. And it was cheaper for my school to use existing city buses rather than providing their own buses and bus drivers for us kids.

Finally, it may even encourage their parents to use the bus more. Currently, taking a bus with multiple children adds up quickly, especially since bus transfers only last an hour and trying to run multiple errands with multiple kids is really, really sloooow.