Don't bother - those in other areas just don't get it. And it's not even about cost - most NY/NJ weddings aren't appropriate for children, period. They are formal, adult events. My cousin wanted his niece and nephews to be in his wedding, plus come to part of the reception - I've never seen such miserable kids in my life. It was a high-end Long Island evening reception.
I am from NJ and was married there. The reception site was $170/plate. We had kids. Not every kid in our extended family/friends, but any kids who were out of state were invited, all kids of bridal party members were invited, and kids we were close to. They had a great time, despite the fact that the church ceremony started at 3:30 and the reception was 6pm-midnight. It was a long day but not one child cried, was bored, wanted to leave, etc. They danced, ran around with each other, played, etc. And none of the adults cared or even noticed.
Maybe it was because I had my own 3 month old daughter at the time so I was aware of people not wanting to leave babies/kids with sitters (or strangers in the case of the out of state guests) for at least 10 hours, but I never considered making a blanket no kids rule for absolutely all guests.
ETA: Not to say that I have a problem with that, if people don't want any kids. That is fine with me. But I just don't think it is fair to say all kids will be miserable, unhappy, bored, etc. Some will, some will enjoy it.
Also, you have to expect many out of towners not to attend. It is unrealistic to expect people to get overnight sitters, etc. for you (not you, but you know, the bride/groom). Aside from leaving your kids for an extended period, it is a large expense, in addition to travel, hotels, and a gift. To expect people to leave their kids and do all of that for you, and be annoyed when they can't/won't, is pretty self-centered IMO.