If you really don't want them, skip the kids all together. When you really don't know the parents well. The kids should mean more to the bride and groom than just cute props. Don't ask a friend of a friend's child to be in the wedding or even a long distant relative just because he or she is cute and the right age. It takes a lot of preparation, patience, prodding, and expense on the parents part to make the day go relatively smoothly.
Kids are kids and they are unpredictable.
If you can accept this and go with the flow then have them in your wedding, but remember not every kid is suited for the job. The hormonal junior bridesmaid who hates the dress and hair while scowling through every picture. The ring bearer who tears down the aisle and rips through the LOUD paper of the new toy that bribed him down the aisle in the first place. Make sure videos and snacks are available and kid friendly. Dress them at the last possible minute.