There are many good reference books to help you prepare your lesson. Of course, always remember that the Bible is your textbook. An exhaustive concordance has been my best reference help for preparing Bible lessons for children, Sunday School lessons and sermons.
The two best concordances are Strong's Exhaustive Concordance and Young's Analytical Concordance. Each of these contain references to the words used in the original languages and their definitions. These books can be invaluable to understanding the original intent of a passage. Most words in our English Bible are translated accurately; many are translated as accurately as possible, but still miss a certain something that could be expressed in Hebrew or Greek that cannot be easily translated into English.
Not only can the process of studying the original language words be fun for you, but it can be fun for the children too. Teach them the meaning of the names of Bible personalities or even the original Hebrew or Greek word for something in your lesson. (Like the Greek word for Gospel, euaggelion, pronounced euangelion, which means, "Good News.")