What is the name of the condition where children have the constant desire to eat, eat, eat?

Prader-Willi. Prader-willi syndrome is caused by a mutation on chromosome 15. It causes the patient to eat almost non-stop and, as a result become excessively overweight. They also often have learning disabilities and other developmental delays.
Bulimia. Bulimia nervosa is a serious eating disorder. People with bulimia nervosa may binge and purge, eating large amounts of food and then force themselves to vomit or do excessive exercise.