21. Where a marriage is annulled on the ground that a former husband or wife was living, and it is adjudged that the subsequent marriage was contracted in good faith and with the full belief of the parties that the former husband or wife was dead, or when a marriage is annulled on the ground of insanity, children begotten before the decree is made shall be specified in the decree, and shall be entitled to succeed, in the same manner as legitimate children, to the estate of the parent who at the time of the marriage was competent to contract.