The manpage suggests if you need the child to be the same as the parent to exec a copy of the parent in the child right after forking.

This doesn't allow reusing initialization work via CoW mappings (like GP was asking about).

	▲	cestith a day ago \| parent [-]
		This is an artifact of doing as the platform is documented to work. You can do what the man page says, or you can do the opposite. The whole article is about how forking on macOS is troublesome.