Vicky asked me to do a follow-up post on this subject because for Mike and me it was a no brainer that kids would be at our wedding. All but one of the children were cousins who had been to big family parties before and knew a lot of people there, which made them comfortable. There was one baby present, but we didn't even think about him crying and "ruining" the ceremony. If he cried, who cares? He's a baby and that's what they do sometimes. (He was perfectly quiet.) Instead of putting pressure on flower girls, we gave each girl the option of walking down the aisle or sitting with their parents. I made them all matching bouquets, but we didn't care what they were wearing.
I think if kids are going to be there, and participating as guests rather than in a separate room with a baby-sitter, it's important that the wedding is a good place for kids. Our ceremony was outside and the grounds stayed open all night for the kids to keep running around in the grass. And kids love dancing, so when it got too dark for them to play outside, they came in and danced. Cost-wise, our caterer offered meals for $10 less for kids under 9, and kids under 2 were no charge. They made slightly less food for the kids in the headcount. Ask your caterer about similar options.
I think that organized ringbearers and flower girls are set-up for failure. I've seen flower girls crash and burn, even the ones who are so super-excited to be flower girls. Until it's time to actually walk down the aisle in front of everyone. It's a lot of pressure for young kids. And it's impossible to keep them clean. Why do you want to worry about keeping kids clean when you've already got plenty of other things to worry about on wedding day? And they squiggle around in pictures, not able to stand still for the photography session.
I think that some weddings are not appropriate for children. Very formal weddings, for instance. And I'm sorry, but Las Vegas weddings. (I just don't think Las Vegas is as family-friendly as they tried to make themselves seem.) And it's up to the bride and groom to make that clear. If you really don't want kids there, be firm. (Although, I was at Vicky's Vegas wedding and I don't even remember any kids being there. Either they were so well behaved they melted into the background or, you know, it was a Vegas wedding and memories tend to be fuzzy about them.)
Have children at your wedding because you love them and you have a good attitude about the silly things they do. Don't have them there if you're a control freak who's worried about children ruining your good time, or because you just don't think a wedding is an appropriate place for children.