The Malleus Maleficarum by Kramer and Sprenger, 1489, Excerpts
Another terrible thing which God permits to happen to men is when their own children are taken away from women, and strange children are put in their place by devils. And these children, which are commonly called changelings, or in the German tongue Wechselkinder
And it can be said that the Divine pity permits such things for two reasons. First, when the parents dote upon their children too much, and this a punishment for their own good. Secondly, it is to be presumed that the women to whom such things happen are very superstitious, and are in many other ways seduced by devils.