Corrected sentence (clear and simple):
Mountains are like huge folds in the Earths rocky crust, and they stay tall for a very long time.
Why this is better — step by step for a 7-year-old:
- I changed "rocky coat" to "rocky crust" because crust is the word scientists use for the Earths outer layer. "Coat" can be used, but "crust" is more correct.
- I added a comma and the words "and they" so its clear that "stay tall for a very long time" is talking about the mountains, not the Earths crust.
- You can also make it shorter: "Mountains are huge folds in the Earths crust that stay tall for a very long time." This keeps the meaning and is a little simpler.
Even simpler version for younger kids: Mountains are big bumps in the Earths crust that stay tall for a very long time.