In Wal-Mart, KMart and most Family Dollar or Dollar General type stores you can get laminated place mats to use not only to keep your table a little cleaner at lunchtime but to help get an educational point across to your child. They make them with the alphabet, numbers, planets, presidents, colors, shapes, cursive letters, addition, subtraction and even multiplication tables. They are about $1 each and are very handy.
If you can't find one to fit your needs, here is a frugal alternative: Create an 8x11 document with the information your child is studying on it, decorate it with colorful fonts and graphics, print it off and laminate it yourself (if you have a machine). This gives you a little more versatility with the information on the placemat and the child can help you create it.