Victory!

She was a rainbow cloud (see earlier discussion here). I told her she could have a very big piece of candy if she wore it.

And we borrowed Elliot's costume from Karen, whose 14-year old son wore it when he was Elliot's age. That's right, Sonic is old.UPDATED TO ADD:Here's a pinnable version of the costume if you're interested (someone asked, so I figured it was easy enough to put up a new head-less pic for Pinterest purposes):

By the way, that's a chopped up hula-hoop inside, to hold the curve in place.