Easy--never create an input field where clicking through on the default value is guaranteed to do something the user doesn't want, irreversibly. That's terrible design.
They'd go from "wtf stop asking me questions" to "you don't need to know that about me" to "wtf I'm locked out?" in about 5 seconds flat. And I don't think there is anything you can do about that. If they implemented a second chance system ("are you sure about that? input a different year or we're going to lock your account") it sounds like that would be another violation of COPPA, defeating the point of the whole ordeal.