Your sentence is almost perfect for explaining to a 4-year-old why they should be careful with butterfly wings. Here's a slightly improved version for better grammar and clarity:
"Butterfly wings break easily, so it’s best to watch them with your eyes, not with your hands."
This version combines the two simple ideas into one sentence using a comma and a lowercase 'so' to connect the thoughts smoothly. Also, 'your eyes' is a bit more direct and natural than 'our eyes' when giving advice to a child.