I was assured when booking our reservation that we did not need to bring anything because "Disney believes you". (My real issue was that we went last May and booked through a
travel agent. She put my DS down as 2, but he didn't turn 2 until after the trip. We are going the same dates this May-- to get in one last trip before he turns 3. I was worried that Disney would have him in the system as being 2 two years in a row and wonder if we were cheating.)
We didn't bring anything last year and had no issues. I had the free little one and DD4 both in matching strollers. DD is tiny, DS is not, so they are about the same size. DH always went through with our DS, and I went through with the 4 yo. They always seemed surprised that I handed them two tickets.
I don't really know how asking for the child's age would help anything. I know this will make me seem like an awful parent, but my kids never knew how old they were before 4-5ish. (They knew colors, numbers, letters, but I just never thought about age

) Also, even if the child said they were older what are you supposed to do if you've already been assured that you don't need proof of age? If my DS mistakenly told them he was 4 and I have no birth certificate with me how do I assure them he's not? If they're going to randomly question you, than they should tell you to bring proof of age.