J.K.
MigreLief is a daily supplement that contains Riboflavin and Magnesium, two things that are shown to help reduce migraine frequency, particularly in people who are deficient of these things.
It also contains feverfew ,so if she is prone to plant allergies keep an eye on her reaction. Get the Children's version if you can. She can take Riboflavin and Magnesium separately if she has allergies.
When a migraine does come on, taking ibuprofen (unless medically contraindicated) as soon as she notices the aura starting and going to lay quietly in a dim/dark room until it passes will help. Keep a sleep mask in the car in case she needs it while you're on the road.