Your sentence is close to being correct, but it can be made clearer and more grammatically smooth. Here's a corrected version:
Original: Black swans hatch grey and change their fluff to black feathers as they grow.
Corrected: Black swans hatch gray and their fluffy feathers change to black as they grow.
Explanation:
- "Hatch gray" uses "gray" as the color adjective (also "grey" is correct in British English).
- Instead of "fluff," "fluffy feathers" sounds more natural.
- Saying "their fluffy feathers change to black" makes it clear the color changes over time.
For a 5-year-old, you might say: "Black baby swans are born with gray fluffy feathers, and as they grow up, their feathers turn black."