This is a double-blind, placebo-controlled, parallel group study in pediatric subjects who have persistent attention impairment following treatment for cancer. This trial has three phases: (1) pre-randomization to establish eligibility, (2) a 12-week, double-blind, placebo-controlled, parallel-group phase with dose escalation based on body weight, (3) a 12-week, blinded extension phase during which all subjects will receive active drug.