It should be explained to the child that their part in the wedding day is important and they must understand what they will be expected to do.
It is a good idea for the baby bridesmaids and pages to attend the rehearsal so that the setting and people are familiar on the day.
On the wedding day, the parents should keep an eye on their child and take responsibility for them whenever possible. An adult bridesmaid should take charge when the parents are not there.
During the service, small treats can rescue most situations - a supply of non-sugary goodies is a good idea.
If there are jobs that the bride wishes the children to do - such as handing out confetti - then the chief bridesmaid or another adult bridesmaid can be briefed in advance to oversee things.