Vyvanse in Children Aged 6 to 12 Years

Inclusion Criteria: * Children ages 6 to \<12 years at study entry * Severe obesity defined as BMI \>/= 1.2 times the 95th percentile at the screening visit * Prior failed attempt of lifestyle therapy per parent/guardian report * Written informed consent of parent/legal guardian and written assent of participant Exclusion Criteria: * Contraindications to lisdexamfetamine, including current or recent (\< 14 days) use of monoamine oxidase inhibitor and known hypersensitivity to amphetamine products * Family history of sudden death or ventricular arrhythmia in any first or second degree relative with any of the following: sudden or unexplained death including sudden infant death syndrome, cardiomyopathy, heart transplant, familial arrhythmia (such as Wolff-Parkinson-White syndrome, long QT interval, or implantable defibrillator). * Any history of fainting or seizure from exercise, startle, or fright * Clinically significant congenital or structural heart disease or arrhythmia * BMI \<1.2 times the 95th percentile at the baseline/randomization visits * Hypertension defined as systolic blood pressure (SBP) and/or diastolic blood pressure (DBP) \>/= 95th percentile (on 3 separate occasions) at the screening OR baseline/randomization visits * Tachycardia defined heart rate (HR) \>/= 120 bpm (on 3 separate occasions) at the screening OR baseline/randomization visits * Current or recent (\< 3 months) use of psychostimulant or sympathomimetic amine * History of chemical dependency * Diabetes mellitus (type 1 or 2) * Current or recent (\< 3 months) use of anti-obesity medication(s) * Previous bariatric surgery * Recent initiation or change in dose (\< 3 months prior) of anti-hypertensive or lipid medication(s) * Thyroid stimulating hormone (TSH) \> 1.5x upper limit of normal (ULN) * Aspartate transaminase (AST) or alanine transaminase (ALT) \> 3x ULN * Fasting glucose \>/= 126 mg/dL * History of mania, schizophrenia, bipolar disorder, or psychosis * Unstable depression or anxiety that has required hospitalization in the past 12 months * Any history of suicide attempt * Columbia Suicide Severity Rating Scale (C-SSRS) with a score of Moderate or Hiogh at the screening or baseline/randomization visits * Children's Depressive Inventory 2 (CDE-2\_ score \>/= 70 (based on parent or child report) at the screening or baseline/randomization visits * Concomitant use of tricyclic antidepressants, selective serotonin re-uptake inhibitors (SSRIs), serotonin and norepinephrine re-uptake inhibitors (SNRIs), lithium, fentanyl, tramadol, triptans, tryptophan, buspirone and St. John's wort at any time during the study * Refusal to use adequate contraception (double barrier method or stable hormonal contraception plus single barrier method, tubal ligation, or abstinence) in girls of childbearing potential * Inability to swallow test capsule (participants will have two opportunities)