A wedding can be an overwhelming experience, especially for a young child. It isn't easy being the center of attention for a crowd of strangers and then having to walk down the aisle. Young children are shy, and easily frightened, and such an enterprise can be tough and maybe even scary.
One of the suggestions I have is to assign an adult to walk down the aisle with your child. If the child does get upset, they can walk the child away from the aisle, and take the pedals or flowers to put them down the aisle
I remember one particular bride who was extremely upset when her four year old didn't strew the flower pedals. She was busy blaming just about everyone, yours truly included. Somehow I should have become a magical Officiant, waved my wand and made it better. However I would have had to leave the podium and it would have disrupted the flow of the wedding.
My second suggestion is lots of practice. Make a game of it. Have your child do a pretend wedding where she is walking down with the flowers. Get her a pretty dress, dress her in it, and make it fun for her to do.
Third suggestion is to come prepared. Make sure you have everything you need for your child's comfort. Is the child potty trained, and do you need diapers? Do they have favorite toys and games to keep them occupied? Make sure they are with an adult they love to be with. If possible naps are extremely important and helpful.
Fourth, build the child's confidence in her ability to be a flower girl. If the child balks at it the day of have a back up plan, or an older child willing to be the flower girl.
Best of luck and have a wonderful wedding!