Getting away with it or not, what kind of example does this following most-likely-real conversation set for your kid:
Parent: "Ok honey, we're coming up to the gates, remember, if someone asks you, you're still 2."
Innocent Young Child: "But <mommy/daddy>, I already had my birthday! Remember, I had the Olaf cake with the sprinkles! We had a bounce house!!!"
Parent: "I know, I know, but just for this trip, tell them you are 2 if they ask."
3 year old child full of questions:" Why?"
Parent: "Just do it please"
Confused ball of sunshine: "Why?"
Parent: "To save mommy and daddy some money"
Sad little child: "Why? I'm 3! I'm not a baby anymore. You told me last week when you wanted me to go in the potty! You said 3 year olds don't wear diapers! You yelled at me that I'm 3 now and needed to start acting like it?!?"
Parent: "Do it and I'll buy you a Mickey Balloon..."