Divalproex sodium
"My 12 year old son has been having migraines for 2 years now.    At first it was only once every few months. However, it progressed to once or twice a week - with vision aura and speech disturbance.  I took him to a Pediatric Neurologist and he said it was likely from puberty.   He prescribed 500 mg of Depakote ER.   It seemed to start working right away.  It has been almost 2 months now and he has only had one migraine now in almost 2 months. Far better than before.  
He hasn't had any of the noticeable side effects - hair loss, weight gain. He takes it before bedtime. It is hard to get him up in the morning but don't know if that is from the medicine or from going to bed too late!"