If you want to make an awesome kid's toy, it should be a thin box that has at least a dozen doors that all have little locks on them. And the doors & locks/keys could be color-coded or have matching shapes/pictures. And when you get the little door open there would be a little box inside. And inside the box would be something fascinating like a button, or a rubber ball, or a tiny monkey. Or you could put all of the keys except for one inside the boxes.