How to explain "nightmares" and "bad dreams" in a way even a child can understand and embrace

So, how to explain bad dreams to the children. I know I have to be creative. I know what they are about: dark things come to remove our darkness. "Not knowing", "Not understanding", "incomplete perception" and "Not believing" are all aspects of darkness we embody at various stages in various amounts. So "dark entities" come to claim them. Those of us who stopped fearing these arrivals of darkness simply lay back and say "Have your way with it, then". It is when we do not understand that we resist, experience fear and sleeplessness. But what would a child be OK with leaving/being taken from them? Consciousness is not appealing because frankly "maturity" they see in adults with all their fear and imbalances is in no way appealing. They will fight tooth and nail to retain immaturity in a number of areas. Ah, but I know what is in the hearts of my own children....my daughter is enraptured by breasts and my son cannot let go of his penis. Conversation is prepared then. "OK, guys, I am going to tell you what your bad dreams are about. Daughter, you want big breasts don't you? "Son, you don't think your penis is going to be that small forever, do you?" The giggle fest has begun! "Well, flat chest and little boy size penis have to go to make room for big breasts and man-size penis and so those things you think are monsters are actually friends to flat chest and small penis, come to take them to a new home". No complaints of bad dreams since.......