The original sentence is mostly clear but can be improved slightly for better grammar and clarity. Here's a corrected version:
"Mangrove trees drink salty water through their roots. They put extra salt into their leaves and get rid of it by dropping these leaves."
Explanation of changes:
- Removed "the" before "extra salt" because we usually say "extra salt" without "the" in this context.
- Kept the rest mostly the same because the original phrasing is understandable and appropriate for young children.
This version is simple to understand and grammatically correct.