"Short of preventing harm to the child, the standard of 'best interest of the child' is insufficient to serve as a compelling state interest overruling a parent's fundamental rights.... To suggest otherwise would be the logical equivalent to asserting that the state has the authority to break up stable families and redistribute its infant population to provide each child with the 'best family.' It is not within the province of the state to make significant decisions concerning the custody of children merely because it could make a 'better' decision."