Bridesmaids were traditionally believed to be decoys, protecting the bride as she made her way down the aisle. Any lurking spirits were expected to take the bridesmaids first, believing them to be the bride. As the bridesmaids were taken away, the bride was free to make her way happily to wedded bliss.
Historically, the bridesmaids surrounded the bride on her wedding day in order to ward-off any evil spirits. In theory, the spirits would be tricked and go after the bridesmaids (instead of the bride).
It is important for your children to be at the rehearsal, especially if they are involved in the ceremony. Children are often junior bridesmaids (or groomsmen), flower girls, ring bearers, or page boys.