Here's a somewhat simple Halloween costume that would also make it easier for drivers to see your kid. Someone posted a video on YouTube of their toddler's STICK FIGURE costume.
Basically, they bought some L.E.D. wire that lights up, then put a circle of it around the kid's face . . . a line down his chest . . . a line down each arm . . . and one down each leg. And the end result is pretty cool.