The sentence you wrote is mostly correct but could be made clearer and smoother. Here is your original sentence:
"In autumn, deer drop their antlers to regrow them in spring."
The sentence is understandable, but it sounds a little like the deer drop antlers specifically to regrow them. Instead, it’s better to explain that deer naturally drop their antlers in autumn and then regrow them in spring.
A better way to say this might be:
- "In autumn, deer drop their antlers, and then they regrow them in spring."
- Or, simpler: "Deer lose their antlers in autumn and regrow them in spring."
Both these sentences are grammatically correct and clear.
If you want to keep it short and clear for a 9-year-old, try:
"Deer drop their antlers in autumn and grow new ones in spring."