Our daughter will be 3 on 2/26 and on our WDW trip 3 weeks ago 2 CMs at the turnstiles asked us if she had a ticket. We just said "no" and that was that. They let her right on in. No questions about her age, why she didn't have a ticket, etc. My daughter knows she's 2, but if asked by a CM she probably wouldn't say anything at all. She's 37 inches so not super tall or anything so I never thought we'd be asked (and we were just there a month before that and never asked) and didn't have any proof. Well, I guess in the room I have her nursery photo in my wallet with her birthdate printed on the picture, but I'm sure I wouldn't have even thought of that.
So I don't think you'll need any proof. Of course, if it makes you feel better bring something, but like I said we were asked if we had a ticket, said no, and they let her in. No more questions asked.