This study is for kids (ages 1-18) with certain blood cancers like acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), or juvenile myelomonocytic leukemia (JMML) that have come back or don't respond to treatment. It tests a drug called imetelstat combined with chemotherapy drugs, fludarabine and cytarabine, to find the safest and most effective dose. The study involves getting these medicines through a needle into a vein and may include additional tests like heart checks and blood samples.

• The study lasts for up to 2 cycles, each lasting 28 days, with follow-ups for 5 years.
• Participants must have tried other treatments without success.
• Women who can have babies and men who can father a child must use birth control during the study.

Joining this study could help find better treatments for these challenging cancers, but it's important to consider the risks and benefits with your doctor.