Grammar check for a 6-year-old: Toucan sentence correction and simple explanation

Your original sentence is grammatically correct.

With short wings and a big beak, a toucan hops between branches instead of flying.

Here is a slightly clearer version you can use:

A toucan has short wings and a big beak, so it hops between branches instead of flying.

Easy steps to understand why this is good:

• Who is the sentence about? A toucan.
• What does the toucan do? It hops between branches instead of flying.
• Why does it hop? Because it has short wings (and a big beak).

Both sentences are correct; the second one just makes the reason a little clearer for young readers.