^ The new requirements are federal vehicle standards legislation. As is common in most regulatory legislation, previous standards are grandfathered (if I'm still allowed to say that), either sine fine (without end) or with a compliance window of several years, often the typical industry service life of the vehicle type.. No, you don't have to install seat belts in a Model T. How US-registered highway coaches are treated remains to be determined, but in many areas, we reciprocally accept US standards (US vehicles operating in Canada do not have to have daytime running lights, for example).
The vehicle equipment standards are distinct from usage, and I think the Ontario Highway Traffic Act and regulations may need some tweaking. The way I read it - and it has admittedly been many years since I had to think about this critically - the driver of a "bus for hire" is responsible for passengers under 16, except where 'child restraint' devices are involved, which seems to leave a gap of several years, which would be an unreasonable burden on the driver. A similar problem would exist if they were to mandate seat belts on school buses. I could be reading it wrong.
I think it is completely reasonable that a parent with a child who requires a booster seat/infant-toddler seat should be required to comply with current similar legislation. There are ample studies that show that the standard 3-point restraint is dangerous for child body dimensions, and the carriers would otherwise carry a significant liability. When I drove for an out-of-town airport shuttle service, parents were obviously diligent in having the appropriate restraint seat for their child, but it was our policy that the parents had to install them in in the van (if for no other reason that only parents can figure the damn things out).