Everybody loves a good wedding! Traditionally, children as well as adults make up the bride and groom's wedding party. Your question refers to the boy or girl who will attend the wedding couple; I think you may be referring to the flower girls, junior bridesmaids/ groomsmen, pages, or the ring-bearer.
This is usually a little girl aged 3-8 years old. She is responsible for scattering flowers from a basket as she walks down the wedding aisle.
These are usually girls aged 8-16 years who are too old to be flower girls. They are junior versions of the adult bridesmaids. Junior bridesmaids often help with miscellaneous tasks at bridal showers or they may even attend bridal fittings with the bride (especially if they are related). Junior bridesmaids also attend the rehearsal ceremony and rehearsal dinner. Before the wedding, they may help seat guests and hand out programs. During the wedding, they walk down the aisle as part of the procession and will stand with the rest of the wedding party. The wedding attire for a junior bridesmaid is a junior version of the adult bridesmaid gown.
Source: Duties of the Junior Bridesmaid.
These are usually boys aged 8-16 years. These boys are usually too old to be ring-bearers. Some junior groomsmen may double as ushers; some may not. These young boys usually accompany the junior bridesmaids down the aisle during the wedding procession. Additionally, some junior groomsmen may escort either the little flower girl or ring-bearer down the aisle (especially if the children are nervous). Some junior groomsmen are tasked with managing the younger attendants during the ceremony and reception. As with junior bridesmaids, junior groomsmen do not attend the bachelor or bachelorette parties.
Source: Duties of the Junior Groomsmen.
A young boy or girl, usually between the ages of 3-7 years old. The ring-bearer is tasked with carrying matching wedding bands on a small pillow or cushion down the aisle. Often, the wedding bands may be fake, in case they are lost.
These are young boys (or girls) ages 6-9 who help carry the bride's wedding gown train (especially if the train is very long). They accompany the bride as she walks into the church and/or down the aisle.