Here are some quick and cute costume ideas that you can put together, one for a boy and one for a little girl.
Beauty Queen
Your little girl can instantly become Miss (whatever state you live in). If you have an old formal, or pick one up at a thrift store, you can cut it down to the right length for her. Take it in if necessary with just a few stitches or with some carefully placed safety pins. Pick up a tiara at any discount store or make one yourself. Use a sparkly headband, but instead of putting it on the regular way, it will sit atop your little beauty queen’s head. Cut out a crown shape from poster board and glue it to the “front” edge (the top when worn the regular way). Decorate with glitter and sparkly faux jewels. Make a sash from wide ribbon. Write on it with glue, cover the glue with glitter, and shake off the excess. A long pair of gloves makes a great finishing touch, but short dress gloves will also work.
Referee
Your little guy will have fun being the referee. Grab some black pants or let him wear jeans if black pants are not available. If you can’t find a striped shirt that will work as a ref shirt, simply use a white shirt and add black stripes with felt or fabric. Use a cheap shirt and just glue the fabric stripes directly to it. An inexpensive polo shirt will work great, and you can cover the collar with black felt as well. Have him put on a black ball cap, sneakers, and give him a whistle. You can put the whistle on a string around his neck so he won’t lose it (although this is not advised for very small children).
Related: Last Minute Costume Ideas