Good job — your sentence is grammatically correct!
Here are two clearer ways to say the same thing (easy for a 6-year-old):
- Simple and clear: A narwhal's horn is really a big tooth called a tusk.
- Similar to your original: The thing that looks like a horn on a narwhal is actually a very big tooth called a tusk.
Short explanation (step by step):
- Your sentence is a complete sentence — it has a subject (what you're talking about) and tells something about it.
- All the words are in the right order, and the meaning is clear.
- Adding words like "actually" can make the idea clearer, and using "narwhal's" (with an apostrophe) is a short way to say "the narwhal's horn."
What is a tusk? A tusk is a very long tooth that sticks out, like the one on a narwhal.