Umm, I'm in Canada too, and it is illegal to have your child out of a car seat or booster seat if they are 8 and under, and it is illegal to have your child in the front seat if they are under the age of 12.
It doesn't matter if your 8 year old would forgive you or not, it is the law. The law states 0-8 years of age and up to 80lbs must be in a car seat or booster seat, So, according to the new laws, your daughter still needs a booster seat if she is under 80lbs and your son still needs his car seat or booster seat until 8 years or 80lbs.
It is based on weight, mostly, because in a 50km/h collision, a 12lbs child becomes a 260lbs child. They do not contain enough fat on their bodies that a seat belt alone would protect them. There are incidences where seat belts have completely cut children under 8 and under 80lbs right in half because they didn't have enough fat on them to protect them.
I will be disregarding age when it comes to car seats, I will be keeping my child in car seats and booster seats until they reach the recommended weight, even if they are 12 years old when they finally reach it.
And by the way, in about 2-4 years, a new law will be coming into place where all kids 10 and under and under 100lbs will be requiring car seats and booster seats.
You better hope that with the way you have your kids in the car, you don't get pulled over and fined, or worse, have your kids taken away.
Edit: There are so many "darn laws" these days because these "darn laws" are meant to protect our children. We know more now about car safety and children than we did back in the 70's and 80's. We had car seats as I grew up, and I grew up in the 80's and 90's.
I'll take these "darn laws" that protect my children any day over an embarassed child.