Yes if they are under a certain weight or height. They have no humour or wiggle room for chidren's safety. So here is the Ministry of Transportation site and the regulations which will answer your(and others) questions:
Safe & Secure: Choosing the right car seat for your child
A good time to remind people not to buy used carseats at secondhand stores or yard sales as often they do not meet safety regulations(after a few years), could have been in an accident, or recalled. Also plastic or belts/parts are cracked/weakened with time and use, etc. Plastic wears down after a few years if left in the heat/cold cycle of an attic for example. The gov't recommends to buy new and never used.
To find out about the carseat recall list-and if yours is on it, call Transport Canada at :
1(800)333-0371 (tollfree).
Most carseats are not installed correctly and they have info at the website about how to do that and where to have it checked for your children's safety.
Have a good trip!