We were in a similar situation on our last trip. My son was about 3 1/2. He is also very small for his age, so he easily could have passed for under 3. I am very frugal and will do everything I possilby can to save a buck. But when it came down to either buying him a ticket or teaching him to lie, I choose to buy him the ticket. I did not think it was a good idea to teach him that it was OK (the appearance of being OK) to lie.