The sentence you gave is very well written! Here is the sentence:
"Hummingbirds flap their wings super fast, letting them hover and fly backwards and upside down."
It is grammatically correct and easy to understand. The phrase "super fast" is casual and fun, which is great for a 5-year-old! If you want to make it a little more formal, you could say "very fast" instead of "super fast", but either way is fine.
So, your sentence is perfect just as it is!