When your child wakes up because of a nightmare, go to her immediately and provide comfort. Reassure your child that it was only a dream and is not real. You may want to turn on a nightlight, and stay with her until she goes back to sleep. It's important that you don't ignore your child's fears - they really matter. But don't get too upset about her fears, either. This will only make her more afraid.
The next day, talk to your child about any problems or worries he may have. This can help stop nightmares. Assure your child that he is safe, that you love him and will protect him, day and night. Remind him of the difference between dreams and reality. Ensure the bedtime routine is one that calms him down. Avoid watching any scary or violent television, or reading any scary stories. A nice story can be soothing, though. So can a bath, a nightlight, singing a song or talking about the good things that happened that day.
If your child is losing a lot of sleep because of nightmares, or beginning to avoid sleeping, consult your child's physician.