True transgenderism or transsexualism is, essentially, a birth defect, in essence no different than being born with club feet or a cleft palate.
Typically, males have a large hypothalamus, and females have a smaller hypothalamus. Sometimes, though, babies are born with the “wrong” size hypothalamus. This results in a person who is biologically male who truly, honestly sees themselves as female, or vice versa.
A simple MRI can confirm whether this child has the wrong size hypothalamus. If so, let her use the girl’s bathroom. If not, then the child is not truly transsexual, but is a cross-dresser and should be made to use the boys’ restroom. Pretty simple, really.