Final answer:
Complete heart block is the most common cause of bradycardia in children.
Step-by-step explanation:
The most common cause of bradycardia in children is complete heart block. Bradycardia refers to a heart rate that is lower than the normal range. In complete heart block, there is a disruption in the electrical signals that control the heart's rhythm, leading to a slow heart rate. Other options listed, such as sepsis, drug overdose, and hypoxia, can also contribute to bradycardia in some cases, but complete heart block is the most common cause in children.