If any person die leaving a child, or his wife with child, which shall be born alive, and leaving a will made when such person had no child living, wherein any child he might have is not provided for or mentioned, such child, or any descendant of his shall succeed to such portion of the testator's estate as he would have been entitled to if the testator had died intestate; and towards raising such portion the devisees and legatees shall, out of what is devised and bequeathed to them, contribute ratably, either in kind or in money, as a court, in the particular case, may deem most proper. But if any such child, or descendant, die under the age of eighteen years, unmarried and without issue, his portion of the estate, or so much thereof as may remain unexpended in his support and education, shall revert to the person or persons to whom it was given by the will.