Your sentence is mostly correct, but it can be made clearer and easier to understand. Here's a better version:
"Hoopoes nest inside tree hollows and make them smell bad to keep animals away."
Explanation:
- "Smelly" or "smell bad" is easier for a 6-year-old to understand than "stinky."
- The phrase "make them smell bad" describes what hoopoes do to the tree hollows.
- The sentence tells us that hoopoes live in tree hollows and keep animals away by making the hollows smell bad.
Great job describing what hoopoes do!