This study will look at how well semaglutide helps children and teenagers losing weight. This will be tested by comparing the effect on body weight in children and teenagers taking semaglutide in comparison to placebo, a "dummy" medicine. In addition to taking the medicine, the child's parent and the child will have talks with study staff about healthy food choices, how to be more physically active and what your child can do to try to lose weight. The child will either get semaglutide or a "dummy" medicine. Which treatment the child will get is decided by chance. Semaglutide is an approved medicine for type 2 diabetes and weight management in adults. The child will get one injection once a week. The study medicine is injected with a thin needle in the stomach, thighs or upper arms. The study will last for about 2 ½ years (132 weeks).