Children can be fickle when they have to do things they are not particularly interested in doing (like walking down the aisle in front of a bunch of strangers or holding another child's hand). So, when planning for your ceremony processional, it's important to remember these things.
The fact of the matter is that kids will be kids, so if your three-year-old ring bearer decides to walk halfway down the aisle and then run off and chase a butterfly, just go with it ... It's entertainment for your guests just having those cute little kids walk down the aisle. If your one-year-old nephew starts screaming right before entering in the arms of your sister, then let your sister know it's OK to turn around and walk the other way.
It's always good to involve kids in the ceremony rehearsal. Not only do the little ones get a feel for their responsibilities on the big day, but they can meet everyone they will be walking with and feel a little more confident when the time comes.
Bribe them (seriously).
My favorite tip (and the one which always seems to work!) is to let them know if they walk down the aisle they can have a lollipop or a piece of candy or their favorite treat. Take note that this should not be colored candy, especially if they are wearing a white dress, but something small and simple. Yes, bribing your child works, and it's OK in these situations!
Designate adult helpers.
Have one parent with the child before they walk down the aisle so that he or she feels safe and secure. Have the other parent seated at the end of the row so they see where they need to walk. Note: If one of the parents is in the bridal party and will be at the altar, then tell the parent to make eye contact with the child so he or she knows where to walk.
Give them props.
The traditional basket of flowers or ring bearer pillow does more than just add to the décor. Having something to carry and a special task makes kids feel like a part of the event. And if the flower girls forget to throw the flowers (it's actually very common), it's no big deal!
Don't force it.
Let them decide if they will hold hands with their co-walkers. If they are not into it, don't force it.
Give them (another) fun role!
Give older children a disposable camera so that they can take photos as they walk down the aisle. It's fun to see what images they take!