Help with nightmares?

my brother in laws daughter is 4 years old and wakes up 2/3 times during the night with nightmares, she has herself scared of bedtime because she is terrified of having a bad drean, resulting in her having bad dreams. this is an ongoing thing for a year or two, she hasnt had a tramatic experience and is generally happy. although cranky at times due to lack of sleep i guess. does anyone have any suggestions on how they can help their daughter sleep better its heartbreaking to see how tired and understandibly frustated they are.