Your sentences are mostly correct for a 6-year-old to understand. Here is the original text with a tiny improvement for smoothness:
"The sun warms up the sand, and the sand makes the desert hot. Once the sun sets, the desert cools down quickly."
Grammar points:
- "The sun warms up the sand" is a clear and simple statement in the present tense and uses "warms up" correctly.
- The conjunction "and" connects two ideas smoothly.
- "The sand makes the desert hot" is also grammatically correct and makes sense.
- "Once the sun sets, the desert cools down quickly." Here, "once" means "after", which is used correctly to describe what happens later.
Summary: Your sentences are grammatically correct and easy to understand for young learners. Keep writing like this!