Correct sentence (simple):
Leaves change colour and fall off trees. This is called leaf fall (or falling leaves).
Why this is correct — in short:
- Leaves is the subject (more than one leaf), so we use change (not changes).
- Fall off trees is a correct way to say the leaves come away from the trees.
- It’s clearer to use two short sentences instead of joining them with a comma: “Leaves change colour and fall off trees. This is called leaf fall.”
- British spelling: colour. In American English you would write color.
Even simpler version for a child:
In autumn, leaves change colour and fall from the trees.
If you want, use this exact sentence: Leaves change colour and fall off trees.