Any parent, even though under twenty-one (21) years of age, may, by an instrument to take effect at the parent's death and wholly written and signed by him or her, or attested by two (2) or more credible witnesses, not including the person appointed as guardian, if not so written, appoint some suitable person as guardian of his motherless or her fatherless child that has not been married, though the child be then unborn and though the child be under some legal disability other than or in addition to minority. Such parent may by such an instrument waive the furnishing by the guardian of bond, inventory and accounting, subject to the approval of the court.