The sentence you wrote is very close to being correct! Here's the original sentence:
Alligators don’t clean their teeth. They can grow new ones over and over again.
It's good, but we can make it a little bit clearer by changing 'grow new ones over and over again' to 'grow new teeth over and over again.' Here is the corrected version:
Alligators don’t clean their teeth. They can grow new teeth over and over again.
This sentence is now accurate and easy to understand for a 6-year-old. It tells us that alligators don’t brush their teeth like people do, but they can grow new teeth many times.