One of the most common child sleep issues I hear about is children who take too long to fall asleep at naptime and/or bedtime. Parents usually report their child is playing, talking, or yelling in their bed instead of sleeping. To try and curb this, many parents move the child's bedtime later assuming they are not yet tired. However, I actually urge you to do the opposite: move bedtime earlier.
When a child is awake too long, they get overtired and their brain releases a hormone called cortisol. Cortisol is the opposite of melatonin. It's job is to keep us awake. People often refer to it as a "second wind". Once the cortisol has been released, the brain and body physically cannot fall asleep.
For example, if your child has an 8pm bedtime, but is regularly playing or screaming in bed until 9pm or later, chances are bedtime is too late. Your child is probably tired by 7 or 7:30pm when melatonin is being released in their brain. However, if they are not put to bed at that time, cortisol is released to counteract the melatonin and keep their body awake longer. Then when you attempt to put them to bed at 8pm, they cannot calm themselves down to sleep. By moving bedtime earlier, you will catch that melatonin release, or sleep wave, and your child will have a much easier time falling asleep as his body is telling him to do so.
Another negative aspect to the cortisol release is that it is long lasting; so a child who is put to bed too late is more likely to wake during to night and wake up too early in the morning. If they are going to bed at a biologically appropriate time, there is a much better chance they will be sleeping through the night until a decent morning wake time (6-7:30am).
This same rule applies for naps. If your child seems to be fighting going to sleep for naps, it is very likely that their naps are simply occurring at times that do not coincide with their natural melatonin release. For children on two naps a day, they should begin within a half hour of 9am and 1pm. For children on one nap a day, it should begin between 12pm and 1pm.