I've read on other threads that cast members can question and even make you purchase adult ticket to enter, if they really think you are intentionally trying to cheat them. However, if you can show "proof" after the fact, they will issue a refund.
I'm in similar situation with DD who will be 26 months and is free. So we had an ID printed up for her with photo from KidzID (along with fingerprints, etc)...we will also carry a copy of her Birth certificate (which doesn't have a photo id). We'd rather have a smooth entrance if questioned, then to waste even 5-10 minutes at Guest Services.
I'm sure you could always call Guest Services prior to your trip to find out the official policy.