Its not the same thing, its apples and oranges.
Any liquor store I have ever been in, there is a sign on the window, "Proof of age required" " No one under 21 permitted to purchase alcohol".
There is nothing at Disney that I've ever seen that says proof of age is required for children under 3 to be admitted for free and under 10 to get a childs rate.
If Disney is going to require proof of age it must be told to you ahead of time. I don't rountinely carry my kids passports or B. C. Such as when a reservation is made and the CM asks your kids ages and if you would state two, the CM could say, "You may be required to show proof of age" But they don't.
Disney barely enforces any other rule they set forth, what would make them start now.