A simple and clear grammar correction of a sentence about how mangrove trees handle salty water, suitable for a 5-year-old.
Your sentence is very good and easy to understand! Here's a little correction to make it grammatically perfect:
Original sentence: Mangrove trees drink salty water through their roots. They put the extra salt into their leaves and get rid of it by dropping these leaves.
Corrected sentence: Mangrove trees drink salty water through their roots. They put the extra salt into their leaves and get rid of it by dropping those leaves.
Explanation: We use those leaves instead of these leaves because we are talking about leaves a little farther away, not right here.