It can be a long day for them and if you don't want your day ruined with children crying or having a tantrum keep these following things in mind;
Photos early - The bride and groom will have got used to cameras and posing for photographs throughout the day. Most wedding day guests know there is the time after the ceremony when they are required for photos. If you have small children who are in the main bridal party as bridesmaids or ushers - make sure their photos are taken first if possible. Before the children get bored or start running around. Often wedding photographers will have a few tricks to photographing children and will be able to cope with this, if they have prior warning.
Entertainment on tables - Depending on the size of the venue and how many guests you want around the tables will dictate the seating plan. One some weddings, children can all sit together but this can lead to fighting and bad behaviour if not monitored. The bride and groom will know the children and how responsible and old they are to sit by themselves. If you are having a table of children make sure at least 2 adults are close by who can be on hand quickly. Think about entertainment and even colouring in activities for small children, if they are sitting with parents to keep them happy.
Childminding Options - If wedding guests are bringing children, chances are they will want them to join in with the celebrations. However, later on in the evening, the parents might want to relax and unwind and if the small child's nap time is right in the middle of the speeches then it might be an idea to see if the hotel offering childminding facilities or services and what the costs are. Parents will need to follow this up for more indepth information as they need to feel comfortable their child is safe, but it would be worth letting parents know this is an option.
Church Seating - This is crucial if you want the ceremony to run as smoothly as possible. The bride and groom will be nervous and if children get bored or want to run around there needs to an adult on hand who can look after them or take them out of the church service, and therefore not mind missing the ceremony. The service can be a long time for any child to sit still, so think about keeping them entertained with any quiet toys or is there a way they can get involved throughout the service. If a baby is crying too much, can they be soothed or if not make sure they are close to an exit to ensure the baby is comforted outside.