Your sentence is almost perfect! Here's the checked and slightly improved version:
"Baby chicks often start out with fuzzy feathers that can easily get wet. So, many water birds, like swans, carry their chicks around on their backs until they are ready to swim."
Explanation:
- I added a comma after "So" because it's an introductory word.
- I replaced "taxi" with "carry" because "taxi" is a bit advanced for a 7-year-old and "carry" is easier to understand.
- "Till" was changed to "until" because "until" is the full form and more formal, but "till" is also correct.
Overall, your sentence is clear and easy to understand!