OOW4CHRIST,
I don't think I know the cheapest airline.
The province and city you are going to is very important. Since you have children, I will advise that you also check the transit (waiting time) of each airline before deciding. For example, if you are going to Alberta and you take a flight to Toronto, you will have another 4+ hrs to Alberta. That means initial transit (?) + transit inside Canada no be small thing for children. For ordinary 8.5 hrs flight from London to Toronto, my 4 year old son requested that the pilot should turn back bcos he was tired.