This study is designed to learn if enlicitide decanoate is safe and effective to treat children and adolescents with heterozygous familial hypercholesterolemia (HeFH) and high amounts of low-density lipoprotein cholesterol (LDL-C) in the blood. The goals of this study are to learn about the safety of enlicitide and if children tolerate it, what happens to enlicitide in a child's body over time, and if enlicitide works to lower cholesterol levels in children more than a placebo.