Going from a mesomorph to an ectomorph means losing weight and becoming skinnier. Some of the weight loss would be from losing muscle. It can be as easy as changing your habits. Do relaxing low intensity cardio workouts and focus on being active. Relax, stay active and avoid exercises that are good for gaining a lot of muscle. The problem with losing weight and being skinny is that muscle makes it easier to keep the fat off. You could end up being skinny with a fat belly or the lack of muscle could make you feel fat.
When I go to the beach most of the people I hear complaining about being fat are female ectomorphs with very little muscle. Changing a 12 year old child's body from an mesomorph to an ectomorph could be easy but it can cause problems later. Skinny children often become adults that have big bellies. If a child wants to be lean they should try to be lean and muscular. They should be athletic.