I have to admit.. I didn't know airlines charge a different fare for children 12 and older.. Up here in Canada, anybody who takes a seat is charged the same price.. and if you have a child under 2, then your child can sit on your lap without getting charged.

I would be above board then.. make sure all info is accurate.. your passports are ready.. and make sure the flight booking info is the same as what is reasonably on your IDs. Takes the worry out of it and gives you a more enjoyable experience.