Here's a fantastic walking robot costume for a small child. Check out the "build notes" after the jump. Just add some water or nerf guns and your kid will be the ultimate ruler of the neighborhood!
http://www.youtube.com/watch?v=7I9v0EdhwlM
L's chicken walking robot costume
http://www.youtube.com/watch?v=bHGtiRo7_gc
Build notes on L's chicken walker