Search the closet and thrift store for bright colored clothes. Choose a simple dress and matching shorts to wear underneath for a girl. Choose a striped shirt and a pair of shorts or overalls for a boy. Colors that will work well are yellow, green, and red. Purchase all clothes in one main color if you are making costumes for group.
Alter the clothing. Cut a slit in the hem of the girls' dresses and thread a soft copper or aluminum wire all the way around the hem. Twist the ends together to secure it. This will make the hem of the dress more cartoon-like, as you can shape it into a circle. Sew rickrack or sequins to the hem of the dress and onto the hems and collars of the children's clothing.
Add bright accessories. Purchase striped leggings or knee socks. Find red ribbons for a girl's hair and red suspenders for the boy. Boys can also wear striped hats or red beanie caps. Add brightly colored striped gloves and red sneakers.
Create exaggerated hairstyles. Add a little spray hair color for boys. Make ponytails with pipe cleaners threaded through the braids for girls, so you can make fun shapes with them. Add big red bows and ribbons to the hair.