Here are three simple ways to say the sentence using different words instead of "reptiles":
- Sea turtles are the oldest living animals with shells.
Why: This helps a child picture the hard shell that turtles have.
- Sea turtles are the oldest living animals that lay eggs.
Why: Many young children know about animals that lay eggs, so this is easy to understand.
- Sea turtles are the oldest living scaly animals.
Why: "Scaly" points to the kind of skin turtles have, which is simple to imagine.