Here are a few simple rephrasings of your riddle made for a 6-year-old. Each one has a small hint and the answer so a child can guess and learn.
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Riddle: I live in the sea but I am not a fish. I have a hard shell to keep me safe. Who am I?
Hint: I might pinch with my claws.
Answer: Crab -
Riddle: I walk on the ocean floor and wear a suit of armor. I am not a fish. What am I?
Hint: I have big claws and a shell that covers my body.
Answer: Lobster (or crab) -
Riddle: I swim in the sea and carry my house on my back. I'm not a fish but I have a hard shell. Who am I?
Hint: I have flippers and a rounded shell.
Answer: Sea turtle -
Riddle: I hide inside a hard home and walk along the sand. I m not a fish but I live in the sea. What am I?
Hint: I sometimes use a shell someone else left behind.
Answer: Hermit crab -
Riddle (short and simple): Not a fish, lives in the sea, wears a hard shell—what am I?
Hint: Think of crabs, lobsters, or turtles.
Answer: Any of those sea animals with shells (crab, lobster, sea turtle)
Teaching tip: After the child guesses, show a picture of the animal and point out the shell and how it helps keep the animal safe.