The major clinical signs used to define AIDS in children under 12 years of age include weight loss, chronic diarrhea, and prolonged fever. These symptoms are often indicative of a compromised immune system.

Option 1, which is a persistent cough for more than 1 month, is not included in the major clinical signs. While a persistent cough can be a symptom of AIDS in children, it is not considered a major clinical sign for the case definition.

Option 2, loss of weight, is a common symptom in children with AIDS as their immune system becomes weakened. This can result in significant weight loss.

Option 3, chronic diarrhea, is another common symptom seen in children with AIDS. It can be chronic, meaning it lasts for an extended period of time, and is often related to gastrointestinal infections.

Option 4, prolonged fever, is also included in the major clinical signs for AIDS in children. Prolonged fever can indicate an active infection or an underlying immune system problem.

In summary, the correct answer is option 1, as a persistent cough for more than 1 month is not included in the major clinical signs used to define AIDS in children under 12 years of age.