Do you have a local art association? If so, contact them for a local artist who would have experience and be willing to come in and help. We did this with a tile mural on a wall of our school. The artist came in and taught/helped the kids paint the tiles, then took them home to be fired and glazed and then brought them back and installed them. We did pay her for her time but she didn't want much and donated most of her time/materials.
Also..Depending on where you live-- in Michigan we have a company that offers coming to the school for the kids to make a tile-- then they ceramic glaze the tile and bring them back-- this gives a nice shiny and protective over on the tiles-- then all you have to do is glue them to the bench.... The company is called Plaster Playhouse-- maybe there's something lie this by you though.
Home depot makes an outside/inside cement bench that you could use for this project-- I think you'll only be able to fit about 20 tiles on the top--- depending on their size to start with and you would have to finish off the top with some cut tiles-- gut this could be very cute... and the top comes off so easy to transport once it's sold...
I saw a post at one time regarding an auction item of a cement bench with ceramic backs that the kids made and was very interested in it. Is there anyone who has any idea about how to do this? I have looked on the internet and found the benches to be very very expense even before the cost of the ceramic tiles to be done by the children. I would appreciate any help that anyone could give me, our auction isn't until March, but this project would require a lot of prior planning/organizing and thought I would get started early on it.