Find a tile design company that will create your tile for you. There are many companies online that offer a range of services from creating individual tiles to murals and tiled keepsake boxes.
Read the guidelines for the specific company, most have a size preference for customized art.
Scan your child's artwork into the computer and save it in the file format specified by the company. If they do not have a preference, create a JPEG file.
Order your tile or tiles from the company. Inform them of your customization preferences, such as color border or size of the tile.