Best advice I can give you is to hold your head high, and if questioned tell the truth. I have had issues in "real life" but not in WDW. Both my DDs are tall (my 8yo extremely so). My 13yo is as tall as me (and I'm 5'6" so not short). My now 8 year old is taller than her 10-11 yo friends on swim team and is also very well spoken. Not a single CM questioned us or batted an eye. I just walked up to the turnstile, scanned my ticket, and went through. She has been going to WDW since around 6-7months, and no one has every questioned me. I would not even mention it. You are not trying to cheat the system. If questioned, then say "her dob is xx/xx/xx"
Enjoy your trip.