Original sentences:
Warm air inside a hot-air balloon makes it float. As the air cools, the balloon comes down.
Are these grammatically correct?
Yes. Both sentences are correct. Capitalization, punctuation, and the hyphen in "hot-air" are all right.
Small suggestion to make them clearer for a child:
Use the word "rise" instead of "float" and use "When" to show the sequence clearly:
Warm air inside a hot-air balloon makes it rise. When the air cools, the balloon comes down.
Why this change?
- "Rise" is a simple word that clearly shows the balloon going up.
- "When" makes it clear that one thing happens after the other.
Final recommended version (simple):
Warm air inside a hot-air balloon makes it rise. When the air cools, the balloon comes down.