Study to Evaluate the Effect of GBT440 in Pediatrics With Sickle Cell Disease

This study consists of four parts, Parts A, B, C, and D. - Part A is a single dose pharmacokinetic (PK) study in pediatric participants with Sickle Cell Disease ages 6 to 17 years. - Part B is a multiple dose, safety, exploratory, efficacy, and PK study in adolescent participants with Sickle Cell Disease ages 12 to 17 years. - Part C is a multiple dose, safety, tolerability, and PK study, which includes the assessment of hematological effects and the effect on TCD flow velocity of voxelotor in pediatric participants with Sickle Cell Disease ages 4 to 17 years. - Part D is a multiple dose, safety, tolerability, and PK study, which examines the hematological effects of voxelotor in pediatric participants with Sickle Cell Disease ages 6 months to < 4 years.