that's a different matter entirely. Very douchey IMO to not allow a single guest to bring a date. That just screams that you're cheap.
here are ignorant and passive aggressive people who would RSVP with the kids included anyway. Then they know they've put you in the uncomfortable position of having to clarify the no kids rule, then they'd feign surprise and say they assumed the whole family was invited.
As everyone says, this is your isue, not theirs. Enjoy your night without the grandson, or don't go.
I do think it's a little weird how the request was made ("please no kids under 12"), but maybe this is a regional thing when it comes to the invitation. If they don't want kids there, don't invite them. You do this by addressing the invitation to "Mr. and Mrs. The Dude" and leaving out anything about "and Family" or "and Grandson". This way if you want to have all your cousins there but not friends' kids, you can put the "and family" on those envelopes and leave it off others. And if there's a teenage kid you do want there, name them specifically on the invitation. Then there's no need for blanket statements.
Cool. So if you're inviting a bunch of ####### idiots to your wedding, make sure to include that line. Do people you invite who are single also need to be told they can't bring a guest to the wedding if they haven't been handed an invitation with "...and Guest" on it?
On a separate note, I went to the wedding of my wife's friend who married an Indian doctor. Very nice affair, but the place seemed too small for all the guests and they didn't have enough hors d'oeuvres. Later we found out that there were dozens of distant family members of the groom who showed up completely uninvited. The staff had to scramble at the last minute to set up more tables for them.