Let's look at your sentences and make them clear and correct for a 7-year-old!
Original sentence: "You can’t find frogs at the poles." This is correct. Frogs can't live in such cold places.
Original sentence: "To live on the poles, animals need to be protected from the cold." This is good too! It means animals must have ways to stay warm.
Original sentence: "Polar bears have warm fur, walruses carry a thick layer of fat and penguins are covered in two layers of down and feathers." This sentence is mostly good, but we can add some commas to make it clearer:
Corrected: "Polar bears have warm fur, walruses carry a thick layer of fat, and penguins are covered in two layers of down and feathers."
Last sentence: "Penguins, by the way, are birds that can’t fly but swim well." This is correct and easy to understand!
So, the full corrected paragraph would be:
"You can’t find frogs at the poles. To live at the poles, animals need to be protected from the cold. Polar bears have warm fur, walruses carry a thick layer of fat, and penguins are covered in two layers of down and feathers. Penguins, by the way, are birds that can’t fly but swim well."
Great job! Just remember to include a comma before 'and' when listing several things.