The sentence you wrote is almost perfect! It says, "Fluffy goose down is used to stuff warm jackets." This means that soft feathers from a goose keep jackets warm. The grammar is correct, and the sentence makes sense.
For a 6-year-old, you could also say:
"Fluffy goose down keeps jackets warm." or "We use fluffy goose down to fill warm jackets."
But your sentence is grammatically correct and clear!