Simple, and lots of fun to eat!
Yes, they could be fancier, but they're still cute! This way I'm willing to give each child a whole apple's worth of butterflies. It takes almost no extra time to cut them this way.
Cut an apple into quarters, then cut the seeds out using a shallow 'V' cut. This gives a curve to the outside edges of your butterflies' wings.
Then cut each piece so you have eight more-or-less-equal wedges.
Cut carefully, starting at what was the center of the apple (the narrow side of the wedge). Cut almost through, stopping about 1/8- 1/4 " from the last bit of edge.
I like to make the uncut part about 1/3 the way down the outside edge, it makes the butterflies look a little more realistic.
Open the two pieces, leaving that skin 'hinge' intact.
See how it's attached still? If you accidentally cut all the way through, just set them back-to-back. They just aren't quite as much fun to eat that way.
This is as fancy as I like to get when I'm trying to get lunch on the table quickly. The antennae are strings off a stalk of rhubarb in the garden.
You could get really fancy and creative here, if you have the time and inclination. Strings of licorice would also work for antennae, but kinda defeats the purpose of the healthy snack. (Besides, you know what happens once a package is opened...)
You could also decorate using raisins, other dried fruit (think colors here), mini chocolate chips, colored sprinkles, even paint on the wings with diluted food color, or a finger dipped in a natural color-- cocoa powder, blueberry juice, turmeric or curry powder, crushed safflower, dried powdered beet or tomato.