Ghosts !! Old sheets/bedding, or find them in a charity shop.
One circle/piece of fabric fitted around the neck and down to the calfs of the legs, two holes for the arms to come through. A pillow case over the head with eye holes, or even one big hole for the face. Easy peasy. No sewing needed. If any minimal sewing was required there'd be no need for anything but tacking stitches, or wonderweb, or even safety pins.
Old pink sheets for a girl ghost, perhaps
Any colour except pink for a boy
I just love love love the home made stuff. Anyone can just go buy an outfit. Home made is special.