A married woman is a joint guardian of her children with her husband, with equal powers, rights and duties in regard to them. Upon the death of either father or mother, the surviving parent, whether of full age or a minor, of a child likely to be born, or of any living child under the age of eighteen years and unmarried, may, by deed or last will, duly executed, dispose of the custody and tuition of such child during its minority or for any less time, to any person or persons. Such surviving parent