Most airlines charge almost adult fares to children over 2. I have twins, who turned 2 last year and looking into ticket prices, really hurts. I know of airlines in South America that charge per kilogram of a passenger's total weight (luggage and personal weight). But unfortunately this fare system hasn't made it to Europe yet.
So my question is, if you are traveling with (small) children, which airlines should have my preference from a budget perspective?