A ladybug costume is a simple design and suitable for any age. But it can break or bend fairly easily. The wings are attached to a bendable wire frame, which is fastened to your shirt. Don't wear it to a rowdy or crowded costume party unless you don't mind it getting damaged.
Make an oval out of wire. It should be about the length of your torso, and wide enough to fit over your shoulders. You can use sculpture wire, or clothes hangers strung together. This will be the support for your ladybug wings.
Cover the wire with red fabric with black polka dots. Either use stretchy fabric, or make it a little loose so that the frame can fit comfortably over your back. Use a sewing machine to attach the fabric or, if you don't have one, use tape.
Attach the fabric oval to a black, long-sleeved shirt. Attach it at the shoulder. You can tape it to the shirt, put straps around the frame so that you can wear it backpack style, or sew it on. Be sure to attach it firmly so that it does not come off, ripping the shirt.
Get bug antennae, such as the ones linked to below.
Wear the shirt, along with something red on your lower half. You can wear a dress, pants, or whatever.
You can wear makeup with your costume if you want to. Ladybugs actually have black and white patchy looking faces, but most people don't go for realism. Red shoes, nails and socks will go well with your ladybug look.
If you are worried about your costume getting damaged, leave the wire frame off the wings altogether. Instead, Sew an oval of polka dot fabric onto your shoulders in the style of a cape.