I based this off old fairy stories like Rumplestilskin and Changeling legends. Fae have always been interested in obtaining people's children for unknown reasons. Perhaps having a mortal child is seen as a valuable link to the mortal world or a tool for future trickery. Maybe it's just in vogue for fae to have mortal pets. But they definitely are bound to follow through with any deal they have made. They are ruled by traditions and rules. Many Fairy stories have fae making bargains and then using the language of the bargain or technicalities to trick the mortal. Fae have little else to do that is more important than tricking mortals. at least that's the impression I get from folklore