If the wedding is the second marriage for the bride, groom, or both, there may be children from a previous relationship. You might consider making these kids a part of your wedding ceremony.
Typical younger kids roles are:
Usually a young boy between the ages of 4 and 8 who has a special connection to the bride and groom. He may be related to them, or just be a family friend. His role is to carry the rings down the aisle to the couple. Usually, the rings are tied to a pillow and then the ring bearer carries this pillow down the aisle.
Usually a young girl between the ages of 4 and 8 who has a special connection to the bride and groom. Perhaps she is a niece or cousin, or a family friend. She follows behind the bridesmaids with a basket of flowers, scattering them down the aisle.
In case your children are grown-ups instead, you may want to consider different roles for them such as the Maid of Honor for a girl, or The One to walk the bride down the aisle, if you have an older boy.
A boy or a girl might also be Readers during the ceremony, reading either a passage about Love or Marriage, or, even better, something written by themselves welcoming the new entrant in the family.
You may become even more creative and assign them other roles during the whole ceremony: announcing the bridal party arrival at the reception, help guests find their seats, have a dance with their parents after their first dance, take care of the floating balloons, remind people to leave comments in the guests book, etc.
Whatever role you decide for them, be sure they feel part of the process, so that they do not feel put aside by the new member, but rather feel the new bond from the very beginning.