How can I reinforce the sleep of a child who already sleeps 11 hours per night?

Based on the information provided, it seems like the child is already getting a sufficient amount of sleep at night. It may be worth trying to establish a consistent bedtime routine to help the child relax and fall asleep easier. Additionally, ensuring a quiet and dark environment can help the body naturally produce melatonin to regulate sleep. If the child continues to have difficulties with sleep, it may be best to consult with a healthcare professional for further guidance and advice.