It may not be a law, but it is a Disney rule. And actually, you would are breaking a law if you sneek you child in when they need a ticket. The law of theft. They could choose to make people prove their child doesn't need a ticket, I don't think they do just to keep the lines moving. Plus then there would be the constant hassle of people who say they didn't know they needed proof, and what would be accepted as proof, and what to do if the parents didn't have proof along with them. I t hink right now it is just easier for Disney to not make people prove how old their kids are.