Additional Study Information: The purpose of this study is to find out whether a new drug, called apixiban, is safe to use in children, and whether it is helpful in children with blood clots. If you choose to have your child be part of this study, s/he will either receive the experimental drug, or he/she will receive one of the usual medications that we use for children with blood clots. Neither you nor your child's doctor will choose whether your child receives the usual medications or the experimental medication.