I wish.
Two simple words we all say on a day to day basis.
We know these wishes are only thoughts and these thoughts are a basis for a desire we all want to have or own. Wealth, fame, and power are the top three wishes magical creatures have heard over the years. The human race has not changed and it comes as no surprise when dealing with adults who make the wishes all for the wrong reasons.
To grant the wishes of an adult one must offer their first born to magical race of their choosing.
It's only fair, right?
To gain something an adult must surrender what was most precious to them.
A babe, which magical creatures adored could only be used as a measure to gauge just how truly selfish and uncaring of the parents offspring was to acquire their wildest imaginations.
A sacrifice that played an instrumental role in making a bargain could only mean the parents weren't interested in keeping their newborn.
The child was probably better off without it's parents to begin with. The ritual for summoning magical creatures involved child endangerment. Only the truly desperate and despicable would ever think to perform and yet are rewarded because of the steps they taken to reach the realms of the supernatural.
It was an unwritten rule and a bunch of backwards logic. This is only the type of knowledge your local folklorist would know, because this was written back in the day. (think dark ages)
A rule is a rule of course which meant in the present day and age a few parents have finally caught wind of the wish granting loophole.
Aside from trying to find a genies lamp.
Children always made legal collateral.