basic saint costume is same as basic Christmas pageant costume for a cheap quickie, use a bathrobe, cord for belt, (hooded sweatshirt underneath for a monk,) towel around the head wrapped with an old necktie for our basic shepherd, OT patriarch, Joseph, apostle, etc., piece of fabric or small table cloth for woman’s veil for Mary, OT woman, Mary Magdelene etc. If you have a little more time, use the basic PONCHO costume, which can be anything you like based on choice of color, trimmings (which can be sewed on trims or with markers), texture. worn over tights and sweatshirt or turtle neck, there is the costume. i.e. buff or tan over brown, fringe the fabric, for an Indian. Light green over dark green with cardboard bow and arrow for Robin Hood–you get the idea.
for a knight (St. George, St Martin of Tours, Joan of Arc), gray on gray, cut shield or lozenge shape from cardboard for coat of arms, red cross on white background for breastplate and shield, cut cardboard sword, cover with tinfoil, hoodes sweatshirt can be made into a helmet with a ski cap over it, and small heraldic decoration on front. Poncho of furry fabric remnant over tights is John the Baptist. Striped poncho over white sweats or PJs is Juan Diego, make a tilma but cutting a magazine picture of OL Guadalupe and pasting on piece of burlap.
If you want to sew there are two basic costumes that cover anything your child can dream up: the basic pajama-jumpsuit, and the basic t-shaped “choir robe” or “graduation gown” shape, long sleeves, tunic etc. again, based on color and texture of fabric and trims, it can be anything you like. Sash or cloak for nun, monk, apostle, priest, brother etc., cut stole of purple fabric for priest stole, veil for nun (St. Theresa, St Rita, all the many nun saints), make mitered hat from 2 pieces of poster board for bishop or pope, get a “saint” book from DRE office or school principal for Saint pictures to get symbols, which are the tipoff for each saint==roses for St. Theresa, rooster for St. Peter, bird for St Francis etc., Crown for King–saint such as- Henry, Louis, David etc., or Queen like Elizabeth, Margaret etc.
You need a saint book with good old fashioned pictures to get the symbols right - raven for St. Benedict in black robe with hood, cross on a chain around his neck. Nuns and friars wear rosaries around the waist. have fun, don’t spend money, let the kids do most of the work. If you can’t sew, you can staple or use fabric glue (iron on seam tape isn’t sturdy enough for kid use, even for one night, and too expensive.)
No need to purchase fabric and patterns and spend a lot of time sewing. A second hand store may have some clothes you could get cheap and convert to a costume, too.