A picture may not always be worth a thousand words, but it will often give more clarity to a theme than any amount of explanation. Although pictures may take as long as the lesson to prepare, I find that the preparation of visuals seems to help you think through your subject and to get it clear in your own mind. In this television age, people are slow to conjure up mental images from the spoken word, whereas a prepared picture visual can do it quickly. A picture also keeps the theme in front of the children even though their minds tend to wander.
Once your visual is complete it is time to practice the whole presentation. You can hold up cards in front of a mirror, or if you are using some piece of equipment like a projector, a chalkboard, or a CD recorder you should rehearse the whole thing as you intend to present it. Make notes as you go along and once you have finished go back and change the wording or improve the visual accordingly. Try to make the material flow naturally from one item to the next by using key linking words or the association of ideas. Work away at it until you feel you cannot improve it anymore. Imagine you are a child listening to it, and think of the reaction he or she might have.
Remember the story of the proud father who took his ten-year-old to see the Indianapolis 500 race. The cars roared round the curve and the boy tugged on his father's sleeve, "Look, Daddy, there's a monarch butterfly!" My advice is enjoy your time with the children, be enthusiastic and your kids will probably enjoy it too!