Also... borrow anything you can, like evening handbags and blazers for the boys. Check out Goodwill and thrift stores. For formal night think "Easter" for the kids... girls in spring dresses with patent leather shoes, boys in blazers with ties. For informal think "school pictures", skirts/dresses and chinos/shirt-no-tie.
On formal nights I wear either a full length gown or a long black skirt with a dressy top. On informal nights I wear either a "little black dress" or black pants (velvet, palazzo, or lace) and dressy top. On casual night I wear a sundress or summer outfit (co-ordinating top/bottom) with flat sandals.
I think Celebrity is a bit more "traditional" and that most people do follow the recommended dress at night. You don't have to spend a lot if you're creative and can find things to mix and match for multiple outfits. And remember... most of the time, less is more. Keep it simple, not flashy. A little black dress with strappy black heels and diamond studs (real or NOT) and you'll look and feel like a million bucks.