Generally, I find weddings to be ridiculously over formalized affairs, with old traditions/language that is archaic now. For example, while it is true that if the invitation is only addressed to the couple, then the kids aren't invited, where else do we really recognize this tradition? Unless you are super upper class and attend many black tie events, how would you really know? While it is true that to follow the traditional rules of etiquette, older kids should be invited separately, they should also be addressed as 'master blah blah', right. Weird language by our standards.
I was married very young, and followed most etiquette rules. However, if we had been married now, it would be very different. BUT, it isn't about how you or I would do it!
Agree with the 'her wedding, her rules' sentiment. However, we would not attend. DH and I define family differently than the rest of our biological family, and we avoid many activities because they don't jive with our values, or are just to difficult with our family, and we include many very close friends when we talk about family, which bugs the bio-family. Conflicts like this are pretty natural when we don't actually share values with family.
However, IF I were planning a super formal event, with a family not accustomed to super formal events, I would expect that I would need to be explicit about my expectations. Putting it in small print on a website isn't exactly Martha Stewart approved.