These mosaics made of pebbles are on the ground and make up a stair along the children garden of the "Colline du chateau" - castle hill. It will take you to the belvedere offering the nicest view on the harbour and the eastern part of the city. They all depict an episode of the Homer Odyssey. (Here, the Cyclop.)
You're sure to have a huge success with children, especially if you can tell them the corresponding stories. (You probably still have time to revise...)