Mosaic work is always a favourite with the children I teach. It ties in well with Roman studies or any culture where mosaic art features prominently.  I usually get the children to design a mixture of ideas on an A3 sheet before starting. This helps to eliminate ideas that may not work and prevent them rushing into something before careful consideration. I order lots of mosaic tiles in a mixture of sizes. Some plain, some more gem like but it is up to you. You can prepare some wood boards as the base for the mosaics but I use cut up cardboard boxes - they are thick enough and work just as well. Get the children to use some pva glue to hold the tiles in place and a lolly stick to space out the tiles so that the grout can be added at the end. Enjoy!