We made my three year old son a homemade British Guard costume for Halloween. I used red material and sewed it into a long, fitted jacket. We trimmed it out with white piping for trim, lots of gold buttons down the front and on the cuffs/collar, and used black and white felt to make the fancy dress sleeve cuffs, shoulders and on the back tails.
They were made as close to the authentic as possible, including the with gold buckle. The pants were black satin with red blood stripes down the side. We used black fuzzy material to make the tall hat. We sewed it into shape and stitched it onto a sock hat so it would stay on better (which is also great if you’re in cool weather!), and used braided gold/red ribbon type stuff as the chin strap.
His candy bag was another hat. The only problem we had was that no one believed we’d made the costume and kept asking where we bought it.