This sounds crazy, but I have the largest two-year-old on the planet. (He wears a 12 shoe) Should I run off a copy of his birth certificate in case I am questioned at the entrance? I would rather have it in my bag than have to pay for admission - whatever it costs. He doesn't talk well, so I don't think he would be able to tell them his age if he's asked. Has anyone else been worried about this?? My husband thinks I'm being compulsive.

They would not risk offending their patrons by calling them liars unless it was so obvious. They even make a joke about it in the Nick Gamelab. Be prepared if you can, but I would not worry about it.
Good thing we flew on a weekday during business hours - I have no idea how I would have proven his age otherwise.
I will now have some kind of id on hand. Like you said - a slip of paper in your pocket or purse is no big deal and you will have it if you need it. Has anyone else ever experienced this?