This flapper costume is a real pip! All you really need for the bulk of the Daisy Buchanan look is a spaghetti strap dress that has absolutely no shape to it. If you can add some fringe to the bustline and hem, that's a plus. For the headgear, an elastic headband will work. Use needle & thread to stitch a few craft store feathers to it.
For more detail, hot glue an old brooch or fake pearls to a piece of felt, then stitch that to your headband as well. Roll a tub of cardboard to make a long cigarette holder, and carry a clutch purse.
The guys can join in with an old-timey Gatsby costume that's just the bee's knees. A nice suit is ideal, or high-waisted slacks with a dress shirt and suspenders. A key element here is hair that's slicked back. You're a Dapper Dan man!
What you'll really need to make this costume the just ducky is some nifty 20's slang. Check out this list of Jazz Age slang. Soon everyone will think you're the cat's meow, even if they have no idea what you're talking about.