A couple years ago I made my daughter a Ladybug Princess costume. The really wanted to dress up to be a ladybug but didn't have the clothes. We contemplated our options and then I made a new dress, of course. I used a sewing pattern for a dress and created a quick tutorial for the wings.
The dress is from Simplicity 1991. It is a very easy pattern though there are a bunch of skirt pieces so it takes a bit of time to come together. The red fabric is costume satin. The black sequin fabric is also costume fabric. Both are available readily at Halloween time.
The wings are super quick. made from this butterfly wings tutorial.
See lots of costume tutorials and ideas here: