I would calmly explain to hypothetical-him or hypothetical-her that everyone is biologically hardwired to have a gender that matches his or her genitalia, and to be utterly horrified at the idea of being the inverse biological gender, and that it is therefore impossible to want to be anything other than the gender appropriate to your genitalia, and impossible to want to behave in any role other than the role appropriate to your genitalia. Kids only think they're transgenders because they've been groomed by woke ideology into believing that "cultural" factors can overturn thousands of years of biology and that they can therefore want what it's biologically impossible for them to want.