Begin by asking the children what they already know about amphibians and reptiles. This will help you get an idea of what you need to focus on. Have children raise their hands one at a time and tell you a fact that they know. Write it on the board.
Focus on one or two animals within the species, such as turtles or frogs. Familiar animals will help grab the children's attention.
Discuss habitats, behavior, what the animals eat and how to care for domesticated turtles, frogs and lizards.
Have children get up and get dramatic with their studies by acting out the behavior of the different animals. Ask the children to be lizards and run around the room acting like one, followed by frogs and so on.
Discuss reptiles as a whole, but do not overwhelm the children with facts. Instead, use simple words and explain them, like "cold-blooded" and explain that this is one thing all reptiles have in common. Give some other examples of reptiles, such as snakes, bearded dragons and alligators.
Read a children's story involving reptiles such as "The Tortoise and the Hare" or "Adventures of Frog and Toad" by Arnold Lobel. The latter also has a corresponding Broadway musical called "A Year with Frog and Toad" with a cast CD available for purchase that you can play in the classroom. After engaging in the material, ask the children what traits they noticed the characters have in common with real reptiles.
Do an art project with your children. This can involve making cutouts of reptiles, drawing the characters in the books you've read or coloring pictures of reptiles.