Because cold sores are viral, the best things are those that help abbreviate the duration of the infection. But, there is no cure, only treatment.
I'd second going to a pharmacist and seeing that is recommended for a child her age. Generally, you want to dry out a cold sore. Products like Carmex are designed for that. Herpecin-L is great (l for lyseine). Abreva is also very good, but her age may be a factor in whether it's the most appropriate product.
Best bet, go to a pharmacy and see what you can learn there and how to prevent future outbreaks:
http://www.mayoclinic.com/health/cold-sore/DS00358