Grammar Check for a Sentence About Penguins for a 7-Year-Old

The sentence you gave is almost perfect! Here's your original sentence:

'A black back and a white belly help the penguin hide in the sea, where the water above is always lighter.'

It's clear and grammatically correct, but to make it even smoother for reading, we can add a small change:

Suggested sentence: "A black back and a white belly help a penguin hide in the sea, where the water above is always lighter."

Explanation:

• Using "the penguin" is okay if you are talking about a specific penguin, but "a penguin" works better when talking about penguins in general.
• Everything else is grammatically correct and the sentence nicely explains how penguin colors help them hide.