In response to the OP:
I can understand your pet peeve and read through your comments on this thread. I also believe in honesty and integrity, and both I and my DH are trying our very best to raise our children to be moral individuals who tell the truth and never feel the need to lie to get anything. At 6 and 4, they know when their birthdays really are, and when we do not throw their actual party on the date of their birthday, they know we are simply "celebrating" their birthday on such-and-such day in order to allow family and friends (ie. those who are important to us) to attend. We also do not go out to dinner or to WDW on their birthday specifically to have strangers aknowledge their day. Maybe it's me but I could care less if a stranger said or worse, sang, happy birthday to me or my children. To me, one word = tacky. If you or others enjoy that type of celebration...good for you.
Now, had I been in the situation that you mentioned at WDW, whether I was celebrating a family birthday or not, I personally could not care less what another table of people was or was not doing, as long as it did not cause my family any physical or emotional harm. When we go out to dine, which is basically weekly, we are out to enjoy dinner and our own company - and truly are not interested in what so-and-so at another table is or is-not celebrating. My family is my priority, not others business. Sure, I think it was rude for the other table to pretend it was someone's birthday or whatever and then have a laugh about it when the waitress left their table but too bad for them. Had I overheard them, I would have probably thought they were lacking class, however it would never had taken away from any celebration that we may have been having. I have certain beliefs and morals, but do not try to push them onto others. We will all have to answer to someone someday.
Have a great time celebrating your DD's birthday at WDW and your other childrens' birthdays 2 weeks or so later, wherever you choose.