Grid based level creator

Hey :) We are a group of university students that have been given the assignment of creating a game for children. The game will be about traffic safety and the parents will be able to make their own levels where they can choose the different obstacles(roundabout,zebra crossings and so on.. We have one week left, and we have been asked to create a simple working sketch of the game, that demonstrates how the map editor works.. What is the most simple way to create a 2d grid based map editor in Unity, where all it has to do is enable the user to drag pictures on to the grid where they will snap together? We are we are hoping to be able to make a small and simple grid(5x5) and there should be a line of images with obstacles in the side, that the parents can drag over to the grid to form the road to school. We got inspiration from this Unity asset, but what we want to create should be way more simple: http://u3d.as/content/bovine-labs/tiler/4WH

We hope the question makes sense..

Thanks in advance