There's nothing wrong with that, just as long as the parent realizes it's just a phase, and the preschool/kindergarten teacher doesn't try to push the kid into something and confuses him.
@PoorSissy Not at five years of age, no child that young understands the concept of gender, unless the parent is as perverted as Jeanette Jennings, trying to push it on the child because the kid wasn't born what they wanted them to be.