For a small child, a car seat in their own airplane seat is the safest way to fly. It just has to carry the right label. On Canadian flights, that's a Canadian seat with the National Safety Mark that isn't marked "Not Certified for Use in Aircraft", or a US seat labelled "certified for use in motor vehicles and aircraft".
Check the label on the exact seat before you buy; the table below lists what Transport Canada accepts from Europe, the UK, Australia and New Zealand too.
Light enough to carry through an airport
You'll be carrying it a long way, so weight matters. A lightweight convertible car seat under 5 kg, with the aircraft label, is far easier to haul to the gate and fits narrow economy seats better than bulky models. A car seat travel cart straps it onto wheels and turns it into a stroller to the gate.
For toddlers: the CARES harness
Booster seats aren't approved for use on the plane. For children of about 10 to 20 kg and up to 100 cm, the CARES harness is the accepted alternative, and it packs into a pouch about the weight of a water bottle. WestJet asks you to contact them after booking so they can prepare your reservation.
If you check it instead
Car seats checked in the hold can get damaged. A padded car seat travel bag with backpack straps protects it and leaves your hands free; inspect the seat after every checked flight anyway.
One rule people miss: to use a car seat on board, your child needs their own seat. Infants on your lap can't use one. WestJet lets you check two pieces of infant equipment free for a lap infant; other airlines vary by fare.