She wouldn't do it.
The flowergirl, who was around eight, suddenly burst into tears at the entrance to the ceremony room as soon as she heard the music swell. Everyone thought she was fine, as she was remarkably mature for her age, so they concentrated on the two younger flowergirls who were already off scattering rosepetals down the aisle.
The bride didn't know what to do. She couldn't comfort the older flowergirl enough to get her to walk down the aisle and the music was running out. The poor groom was sweating bullets.
In the end the girl's mother came out of the ceremony to give her a cuddle and tell her everything was going to be alright. The bride walked down the aisle and the rest of the day went off without incident. And the flowergirl recovered, once the pressure was off, and enjoyed the rest of her day.
You just never know how kids are going to react during the wedding day. They don't really understand what is going on and it can be overwhelming. That's why I think it's better to have low expectations of them and consider it a bonus when they do perform their role well.
In my experience the best wedding photographs of children are the candid ones of them running around, rather than posed photographs.
Look at this little lad. If I'd told him to stop, stand up straight and face the camera, would I have captured the same smile, the same sense of fun, his personality? It's unlikely.
That's just one of the reasons why I prefer documentary wedding photography rather than traditional wedding photography.