Yes I will take this seriously and never ignore the fear feeling. You can ask her why, where, how they see the monster. There is a good trick where you get a "monster spray" and put near to them. It is just a spray bottle consists of water for the kid to spray whenever he feels scared. In addition, what I do for my kid is to pray with her before bedtime and assure her that Jesus will be by her side when she is sleeping and protect her. I taught her a hymn so she will sing it whenever she feel scared. If all the above methods failed, I will sleep with her until she has settled down.