Magic is powered by children’s wishes. And those of adults too. Look at how many stories there are in which magic comes into a person’s life and it turns his world upside down. If you look, you’ll discover that someone has made a wish. Cinderella wished to go to the ball. Aladdin had three wishes. Charlie wished to visit the chocolate factory. There’s always a wish. Sometimes it comes in the form of a prayer either to God, Santa Claus, or some other being, but there is always a wish.
The thing about a wish is that it acknowledges that the wisher has no or limited ability to accomplish what he would like to happen. Charlie was locked out of the chocolate factory and had no money to buy candy bars. Without the help of a higher power, he would have had no way of getting inside. I suppose wishes in stories are a parallel to prayer in real life.
But wishes are fleeting. They have power for a while, but stories are about character change. The higher power in the story gives the character what he wishes for, but as God often does with us, the higher power requires the character to learn from the experience. The character must accomplish the final goal without magic. In Angels in the Outfield, the angels were not permitted to help with championships. What that accomplished is that it forced the character to find another way to accomplish the same thing. In Cinderella, the magic wears off at midnight and all that’s left is a shoe. The result is that Cinderella and the prince must find each other without the aid of magic.
Magic is a fun thing to use in a story, but it is like a dream. At some point, we wake up and we are in the real world. If the character can’t learn to do what he wished to do without magic, his wish is lost forever. But if he learns the lesson, the magic can safely return.