If payment of the special death benefit is stopped because of death of the surviving spouse or death, marriage, or attainment of the age of 22 years by a child before the sum of the monthly payment made, exclusive of the annuity derived from the accumulated additional contribution of the deceased, equals the basic death benefit, a lump sum equal to the difference shall be paid to the surviving children of the deceased member, share and share alike, or if there are no children, to the estate of the person last entitled to the allowance. In that event, the accumulated additional contributions of the deceased, as they were at his or her death, less the annuity paid as derived from those contributions, and plus interest credited to the accumulated additional contributions, shall be paid in the manner provided in this article for the payments of amounts due in the absence of a designated beneficiary.