I would always suggest not letting your child watch any form of devices at least a good hour before bedtime, no devices including TV, tablets, phones as the light from these sources (blue light) can disrupt your sleep-wake cycle. This can prevent them also from falling asleep in a relaxed way, it can also cause disturbed sleep during the night because the brain is still over stimulated from the blue light, 

Ensure you create a good routine, so they are going to bed the same time each night, a chilled bedtime settle, take the opportunity of spending time to read 1-2 books with your child, talk about their day. It’s good they fall asleep unaided on their own it is the time when they have their own thoughts and relaxation before drifting off.

Poor sleep habits 

• Irregular sleep schedules
• Not relaxing or winding down before bed
• Taking naps out of their nap widows or too late in the day if no longer of nap age
• Being too active before bed
• Using electronic devices before bed