My thought on this is that they should mark the child's gender as to what it is on their birth certificate. As the child grows they can of course choose to be what they want to be, that is not for me to judge. But to tell them from the start that it is unknown, what gender they are, is kind of saying what they were born as was not normal, and it is completely normal. Bring them in to the world as a boy or girl, whichever it is, and then give them the freedom to decide if they want to change or choose a different gender.