Clinical Neurology and Neurosurgery reports that in a double blind placebo study of 24 children with cerebral palsy, botox was an effective treatment for drooling although there was a high drop out rate in the study. 

Reference:  Ali H. Alrefai, Samah K. Aburahma and Yousef S. Khader (2009) Treatment of sialorrhea in children with Cerebral Palsy: A double-blind placebo controlled trial Clinical Neurology and Neurosurgery Volume 111, Issue 1, January 2009, Pages 79-82