Nothing in any ante-nuptial contract of marriage executed after the commencement of this Act shall operate so as to exclude, on the occurrence of the death of either party to the marriage, the right of any child of the marriage (or of any issue of his coming in his place by virtue of the last foregoing section) to legitim out of the estate of that party unless such child or issue shall elect to accept in lieu of legitim the provision made in his favour under the contract.