Gender fluidity conveys a wider, more flexible range of gender expression, with interests and behaviors that may even change from day to day. Gender fluid children do not feel confined by restrictive boundaries of stereotypical expectations of girls or boys. In other words, a child may feel they are a girl some days and a boy on others, or possibly feel that neither term describes them accurately. It is best to ask people who are gender fluid which pronouns they prefer.
See gender-expansive.
REFERENCE: Gender Spectrum Guide to Gender Terminology