I don't think YWBU in this case, but I think it gets dodgy if you make a child older, because those age restrictions are usually about safety, not price. I also think it's a bit dodgy wrangling train or bus fares, because no matter how old you are, you are still using the service to its full. I remember my own mother being gutted over train-travel when I turned 16. She never bought me an under 16s ticket, though.
Also, re the "tell them she had her party yesterday and her birthday's tomorrow, but you told her it was her birthday on her party day" - I have worked on the gate for attractions (Monkeyworld actually - hello Monkeyworld lovers!) and that is just a little bit bonkers... Why would you say you lied to your own child about her birthday!? Just say it was her party yesterday and she's getting confused about the date, not that you lied to her about it!
And FWIW, as an ex-gateperson, the less complicated your lie, the better it sounds. If your kid pipes up it's age, just laugh it off or say something stupid. The ones with loads of background sound dead fishy.
(I'm now not associated with Monkeyworld
)
Hope your DC had an awesome birthday, OP!