Felt really is my best friend at Halloween. Without any tedious hemming you can achieve a very finished looking costume. When shopping for felt be aware that there are a lot of different types -- you can find 100% acrylic felt for as little $7 a yard, and 100% wool felt (which is really gorgeous, but not necessary for a kids costume) for about $60 per yard. This costume took less than an hour to make and Bea was quite happy with it. I was too, except for the ears, which are a little too coquettish for me.
To make this costume I had Bea lay down on a large piece of paper and I traced around her body to gauge the size. Then I cut three felt pieces: 1 in a standard tank top shape for the front, and two pieces with a curved edge seam to create the rounded back (see sketch below). NOTE: The straps in the sketch are too short...be sure to make them longer so you can adjust the size easily.
The back of the bunny did not stay rounded out on it's own so I tacked a small square throw pillow inside (photos below). The tail is a scrunchy wash ball from the drug store. Craft stores sell little squares of faux fur which we used along with some white felt for the ears. If I did this again I would be sure to set the ears apart a bit for a different look.
p.s. I intended to put a button and button hole on the straps but ended up just using safety pins instead.