Great sentence! Here are clearer ways to say it:
- Simple correction: Leaves change colour and fall. This is called leaf fall.
- Another way: When leaves change colour and fall, we call it leaf fall.
Notes for a 6-year-old:
- It is nicer to use two short sentences: first say what happens, then name it.
- "Colour" is the British spelling. In American English, it is "color."
So your idea was right — just a small change makes the sentence sound natural!