Here are two clear, simple ways to say it. Both are correct for a 6-year-old.
- Option 1: When sand gets inside, the oyster turns it into a shiny pearl.
- Option 2: When sand gets inside, the oyster coats it to make a shiny pearl.
Notes: In Option 1, turns shows a change that happens. In Option 2, coats describes covering the sand with layers to make the pearl. Both sentences are grammatically correct and easy to understand for a young learner.