The instructions for yachatz in every hagadah that I have seen say to break the middle matzah in two (though, the thin paper hagadah I used in elementary school said, "break in half") and put the bigger piece (the school hagadah said "bigger half". It was only years later that I realized this is impossible!) away for the afikoman.
No instruction I have seen said anything about hiding it or having kids look for it and then "bargain" for a present or prize in order to return it.
So, when and why did this custom originate? Is there any Rav who may have been against this custom, perhaps, because of the scenario I mentioned or some other reasons? Personally, I felt extremely uncomfortable with the price bargaining, esp. on a Yom Tov. It sounded like the afikoman was a business deal.