Children can have difficulty learning math facts for many different reasons. For some, they need to use math manipulatives to understand the concepts underlying math computation. For others, they understand the concepts but just don't have the automatic retrieval skills necessary for true math fluency.
Several programs address the common problems children experience in learning basic math. I like the Structural Arithmetic series from Educators Publishing Service (1-800-225-5750 or http://www.epsbooks.com). It stresses the use of math manipulatives to anchor basic math concepts. I also like the Strategic Math series by Susan Peterson Miller and Cecil D. Mercer, published by Edge Enterprises (PO Box 1304, Lawrence, Kansas 66044). Both series include games to reinforce math concepts and increase speed of retrieval of facts. Great Leaps has just published a new program for increasing speed and fluency in math facts that you might find useful. You can reach Great Leaps at 1-877-GRLEAPS or http://www.greatleaps.com.