Correct sentence: Tough, narrow leaves help an acacia tree lose less water.
- Leave → leaves: Use leaves because we mean more than one leaf.
- Add a comma: Put a comma between tough and narrow to make the two describing words clear: tough, narrow.
- Say "acacia tree": Saying acacia tree is clearer for a child than just acacia.
- Keep/lose water: "Lose less water" is correct, but you can also say "keep more water" which is easier to understand.
Easy version for a 6-year-old: Tough, narrow leaves help an acacia tree keep more water.