Section 20. If a testator omits to provide in his will for any of his children, whether born before or after the testator’s death, or for the issue of a deceased child, whether born before or after the testator’s death, they shall take the same share of his estate which they would have taken if he had died intestate, unless they have been provided for by the testator in his life time or unless it appears that the omission was intentional and not occasioned by accident or mistake; provided, however, that no such child or issue shall take any share in any real property in the testator’s estate unless a claim is filed in the registry of probate by or in behalf of such child or any of such issue within one year after the date of the approval of the bond of the executor.