Playsilk wings like these http://www.atoygarden.com/index.cfm?...Product_ID=853
are easy to make and would make good bat wings.
Measure your child from wrist to wrist to get thier "wingspan" and down their back to get an appropriate length. I didn't have a playsilk so I just hemmed a piece of fabric with the appropriate dimensions.
To turn it into wings add a ribbon loop at each of the top corners, big enough for you LO to get thier hand through. Run a couple of lines of running stitch up the centre of the fabric and gather. this makes your wing shapes. Now sew a length of ribbon at the top of the gathering. This needs to be long enough to cross over you LOs chest, round their back and tie at the front.