This study investigates how well an investigational medication works for patients with juvenile myelomonocytic leukemia (JMML) that has returned or did not respond to treatment. JMML is a rare type of blood cancer that affects young children. The purpose of this study is to determine how well the investigational medication can reduce the cancer in children with this condition.

Participants will undergo certain procedures to evaluate their response to the investigational medication. This includes regular medical check-ups and tests to monitor the effectiveness of the treatment and any side effects that may occur.

• Who can participate: Children with juvenile myelomonocytic leukemia that has come back or does not respond to treatment may participate. They must have fully recovered from prior treatments, not be refractory to blood or platelet transfusions, have undergone at least one cycle of prior therapy, and have a performance status score of 50 or higher. Participants should not be pregnant or breastfeeding and must not have certain medical conditions or be on specific medications.
• Study details: Participants will take trametinib, an investigational medication, to see if it can reduce their cancer. They will be monitored for any changes in their condition and for any side effects.