What do you call a half boy half girl person?

You may be referring to someone who is gender fluid, that is someone who can be a man or a woman (or another gender, without gender, a mix of gender) which can include being exactly half boy and half girl in how they present or how much time they spend as each. Some people may just refer to themselves as gender queer without further definition, but it all depends on what exactly you mean by 'half boy half girl'.