My mother said I never should
Play with the fairies in the wood
If I did, she would say
You naughty girl to disobey
Your hair won't grow, your shoes won't shine
You naughty little girl, you shan't be mine!
This poem refers to a little girl going to play with fairies, and being… replaced.
A Changeling is a creature found in Western European folklore and folk religion. It is typically described as being the offspring of a faerie, troll, elf or other legendary creature that has been secretly left in the place of a human child. A human child might be taken due to many factors: to act as a servant, the love of a human child, or malice. Most often it was thought that faeries exchanged the children. Some Norwegian tales tell that the change was made to prevent inbreeding: to give trolls and humans new blood, humans were given children with enormous strength as a reward. Often, the faerie child left in its place would be a source of mischief or disaster.
Faeries are creatures of myth and folklore. They are the namesake of fairy tales, warning stories to scare children into obedience. They are in our fears and dreams. They are not real.
That doesn't mean that they don't exist.