People have strong opinions about almost everything when it comes to weddings - nothing more than whether or not kids should be in the wedding ceremony. Some think that children should never be involved, while others think that there is nothing cuter than little flower girls and ring bearers prancing down the aisle. The important thing is to do whatever you feel is right.
If you would rather not have children in your bridal party, then that is totally fine! Don't let anyone pressure you into doing otherwise. If there are children in your life that are special to you, then honor them (and their parents) by inviting them to be part of the wedding.
If you do decide to have children in your ceremony, then remember two things:
1) Especially if your flower girl and/or ring bearers are under the age of 6 or so, consider having them walk down the aisle and then sit with a family member during the ceremony. Even if your ceremony is only 15 minutes long, that is still a pretty long time to expect someone so young to stand up in front of everyone and behave themselves.
2) Have someone designated to be in charge of watching the kids before, during, and after the ceremony. Family members or a trusted babysitter are usually good choices for this task. You and your bridesmaids are going to have a lot on your plate, so passing this job off to someone who can concentrate on the little cuties is a great plan.
If you do decide to have the kids join in, you just might get some adorable pictures.