You'll need a large black robe or cape. Button or tie it down the middle, so what you wear underneath doesn't matter.
You can make a Death eater mask that looks like the ones from the movies, or you can come up with your own design. You can make a mask that fits your face perfectly with just a little newspaper and some glue. After you've made your mask, it's easy to paint on your death eater designs.
Again, the robe pretty much takes care of any other clothing issues. Wear a pair of black slacks, or make your own simple black drawstring pants.
Go with a pair of hefty black boots. Even black sneakers will work in a pinch, and for kids.
Wear a pair of black leather gloves. Any black gloves will do, really.
Slick your hair back with gel and keep the hood of your robe up to draw attention away from your hair.