Single Flower Girl
Generally age 3 to 7, she precedes the bride down the aisle, either scattering petals or carrying a mini bouquet or flower basket.

Multiple Flower Girls
Same duties as above; a larger group often includes older girls who sub for bridesmaids.

Ring Bearer
A boy, 3 to 7, who walks the aisle before the flower girl, carrying stand-ins for the rings on a cushion.

Pages
At formal weddings, boys age 4 to 7 who hold the bride's train as she walks down the aisle.

Junior Bridesmaids and Groomsmen
Children age 8 to 12 who perform the same duties as their older counterparts (short of attending bachelor or bachelorette parties).

More Options
Ushers, readers, guest-book attendants, candle-lighters, bubble blowers, and others.