Most children enjoy dressing up and pretending to be someone/something else. This is a common part of play among preschool and early elementary kids. Halloween is a great opportunity for kids to use their imagination.
In general, costumes for very young children should be of familiar characters, people or objects and not cover or alter the face. By second grade, most kids understand the difference between reality and fantasy. Watching your cousin put on a scary mask can still be frightening for some of 6 year olds. If Ursela and the Wicked Witch of the West are characters your child dislikes, be proactive in helping your child avoid interactions with big kids that might enjoy werewolves and mummies.
Thirty years ago, almost all Halloween costumes were home made. Consider reviving the tradition. A paper grocery bag can be cut into a vest and decorated as a police officer, native american, cowboy, etc. Paper plates can be cut and safety pinned to the back of a shirt as wings for a fairy, angel, insect or princess. Parents can help add glitter to the edges. Paper or fabric letters can be pinned to a shirt to create an athletic jersey. Spots can be added for a cow or dog. Stripes can be added for a tiger or bumble bee. The school glue that advertises it is washable is an easy way to attach ribbon for stripes and they conveniently come off in the wash.