When planning your wedding ceremony there are a few options on how to include your new blended family.
The options vary depending on the age of your children. If the children are around 6 and younger one of the more popular options is the sand ceremony. This is a symbolic joining of the family. You each have a container with different colored sand and you pour the sand into one container. I've seen sand, stone, jewels and other items included in the ceremony as a way to personalize it.
With children of any age you can write a vow to them. This statement is usually made after your vows to each other. You would write a statement about your love for each other and how that will strengthen the bond you will have as a family and how your love will flow from not only each other but to all children.
With older children they could take part in the ceremony by being a Jr. bridesmaid or Jr. groomsman or they could be assigned duties to help with seating or placing of items for the ceremony.
A tree planting ceremony is a lovely way to include the entire family in a memorable ceremony. You would each pour a container of sand into the potted plant/tree to plant your roots as a family.