It is very common for pre-school age children to reverse their letters and numbers. By itself this is not a sign of dyslexia. Although a few children may start reading and writing very early, most kids are not ready until they reach school age. In other words, it's normal for children under the age of 6 to have difficulty with reading or writing tasks.
Parents can assess whether their child is potentially dyslexic by having a good understanding of the positive as well as negative traits associated with this learning style. Dyslexic children often show a high interest in working with puzzles and blocks, or are highly creative and imaginative. The book The Gift of Dyslexia summarizes the traits of the potentially dyslexic child.
We also have a kit for parents who want to work with children aged 5-7 to help build pre-reading and early reading skills. This kit, the Davis Young Learner Kit for Home Use, includes the same techniques that we use in our primary level classroom reading program, Davis Learning Strategies.
Another good book is "Your Child's Growing Mind" [ISBN 0767916158 (amazon.com, search)] by Dr. Jane Healy. This book has wonderful information about the growth and development of young children's brains, and is full of suggestions of enriching activities for youngsters. It is far more important to focus on pre-reading skills than to try to teach reading or writing to a child who is not ready.