This question is asked a lot. I mean.. A LOT. If you are getting married with a child, it's only natural that you want that child (or children) to feel as though they can participate in the wedding. It makes it special for both the couple AND the child. My advice? Incorporate a Unity Sand Ceremony into your wedding. The Weddingstar Heart Shaped Sand Ceremony Set even comes with an accessory vase for your child to help pour.
Adding a child (or as I mentioned, children) into the sand ceremony set symbolizes the "blending" of family into one solid unit. The final product is a keepsake that can be cherished forever. Weddingstar, in fact, offers the accessory vase as an additional product so you can purchase as many as you need.
Another added bonus is that you can purchase as much sand as you'd like, and you can pick colors to coordinate with your wedding theme or color scheme.
I'd also appoint my child with some duties that make him/her feel special. For example, they can show the guests to the dance floor or help take coats (if they want to). While this is a very special day for the husband and wife, it's also a special day for the children that are included, so make it fun and memorable for them too!