A child's room bears a dual responsibility. It is a fun, personal space, but it's also a place for quiet time, study and sleep. A child should feel her room is her own, but the space should also be treated with respect. If clutter and messes are a problem, it's easier to provide a solution than to have the same arguments over and over.
Durability and ease of cleaning are the watchwords, but this must fall into harmony with the natural uses of the space. A child who is outdoors all day doesn't require the same environment as a budding artist who prefers time in the sanctuary of her room.
While it's good to let the child guide your choice of accents and art, remember to save some of those personal objects for the rest of the house. Displaying art your child loves in common areas or adding her books to the family library shows that you respect her tastes.