Grimm has collected a list of authorities who speak of the pied piper event as an historical fact. Sabine Baring-Gould tells us this in a footnote, in his book, Curious Myths of the Middle Ages. In the same footnote, Baring-Gould lists Thorpe, Northern Mythology, Volume 3 as affirming the same.
The full title is, Northern Mythology: Comprising the Principal Popular Traditions and Superstitions of Scandinavia, North Germany, and the Netherlands. Benjamin Thorpe compiled it from original and other sources. The year of publication is 1852. Whatever else you might think about Google, I can’t thank them enough for having digitized and made available so many old, out-of-print books.
“A person once saw a female dwarf going across a field with a stolen child.”
“The grandfather of a watchmaker still living in Hohn, was when a boy one day tending cows in a neighbouring field, and to protect himself from the rain, had thrown his father’s large coat over his shoulders. While standing under a tree, he found himself on a sudden surrounded by a multitude of underground folk, holding each other’s hand, and thus forming a circle about him. They told him they were about leaving the neighborhood, and that he should go with them.”
The Subterraneans Lick Up Milk: “About seventy years ago little underground beings were seen in many farms in the Wilstermarsch, who did little more than accompany the maids and men home in the morning after they had been milking, and sedulously lick up the drops of milk that had been spilt.”
Today we would call such items from Benjamin Thorpe’s book “conspiracy theories.” Back around 1852 they called them “superstitions.”
What really happened on June 26, 1284, in the German town of Hamelin? Forget “revenge of the rat catcher” – that part is an embellishment. It is not until the year 1384 that we get a firm report, this one from the town’s chronicle. The entry for that year states: “It is 100 years since our children left.” 
The next reference comes from a Latin chronicle from the German town of Lunenberg:
Here follows a marvellous wonder, which transpired in the town of Hamelin in the diocese of Minden, in the Year of Our Lord, 1284, on the Feast of Saints John and Paul. A certain young man thirty years of age, handsome and well-dressed, so that all who saw him admired him because of his appearance, crossed the bridges and entered the town by the West Gate. He then began to play all through the town a silver pipe of the most magnificent sort. All the children who heard his pipe, in the number of 130, followed him to the East Gate and out of the town to the so-called execution place or Calvary. There they proceeded to vanish, so that no trace of them could be found. The mothers of the children ran from town to town, but they found nothing. It is written: A voice was heard from on high, and a mother was bewailing her son. And as one counts the years according to the Year of Our Lord or according to the first, second or third year of an anniversary, so do the people in Hamelin reckon the years after the departure and disappearance of their children. This report I found in an old book. And the mother of the deacon Johann von Lude saw the children depart. 
The “Calvary” mentioned above is the name of a nearby mountain. The townspeople said that a man had been seen, possibly a musician, wearing clothing of many colors and possessing a pipe, which he played in the town. Whereupon the children in the town ran out as far as the mountain, and there they all disappeared into it. 
The good people of Hamelin clung to their story. This was no “conspiracy theory” (a term used by some today like magic words which cause troubling questions to vanish from their minds.) Even in the 16th century the townspeople would recall the event, such as when a new gate was built along the town walls. At this gate they inscribed the following message: “In the year 1556, 272 years after the magician led 130 children out of the town, this portal was erected.” 
Forget mundane explanations. The Hamelin event of 1284 is more akin to Indrid Cold and Point Pleasant, West Virginia, or to John E. Mack, MD and his abduction narratives. Mack studied the subject and evaluated it non-judgmentally: “I take them [alien abductions] seriously. I don’t have a way to account for them,” he stated. 
It was, to Dr. Mack, a compelling powerful phenomenon that he couldn’t account for. Before he died in 2004, his research had broadened into the general consideration of the merits of an expanded notion of reality, one which allows for experiences that may not fit the Western materialist paradigm. 
Experiences which may not fit the Western materialist paradigm… Like a person who once saw a female dwarf going across a field with a stolen child. Like a boy who found himself on a sudden surrounded by a multitude of underground folk. Like the Subterraneans who lick up milk.
——- Sources ——-
 “The Pied Piper of Hamelin: A Medieval Mass Abduction?”, by Medievalists.net, December 7, 2014. http://www.medievalists.net/2014/12/07/pied-piper-hamelin-medieval-mass-abduction/
 “John E. Mack”, Wikipedia, June 7, 2015.