1. Age: greater than or equal to 3 years and less than or equal to 18 years of age at the time of study enrollment. The upper age limit is in place because early childhood and puberty are considered to be the greatest risk for disease progression, and where sorafenib may have the most benefit. In addition, an important objective of this study is to characterize the pharmacokinetics of sorafenib in the pediatric population since it has been well studied in adults.