With increasing life expectancy the risk of a child predeceasing has increased. To compound this problem there is also a risk that both a child and that child’s own child will predecease the testator but will leave issue.
To add more complications there are risks that a potential beneficiary (either child or remoter descendant) will not predecease the testator but will fail to reach the qualifying age – say 25 and will leave issue, who may or may not reach the qualifying age.
The learned authors of Williams on Wills at 10 Old Square, Lincolns Inn considered these issues in detail between the 9th and 10th editions. The wording of the tenth edition has been substantially altered to reflect their thoughts.
For example, Williams on Wills, Volume 2 Precedents and Statutes, Section C Precedent C2.1 : Reciprocal Wills giving entire estate to spouse absolutely, with alternative gift to children and remoter descendants has been changed:
Beneficial trusts for issue
7. My trustees shall hold my residuary estate and the income thereof upon trust for such of my children [by my [wife/husband]] as shall survive me and attain the age of [18/25] years and if more than one in equal shares Provided that if any such child shall die before attaining a vested interest under the foregoing trust leaving a child or children living at or born after my death who shall attain the age of [18/21] years [or marry or form a civil partnership under that age] then such last-mentioned child or children shall take and if more than one equally between them the share of my residuary estate which their deceased parent would have taken had he or she lived to attain a vested interest.
New Version in the Tenth Edition
Beneficial trusts for descendants if the trust for the spouse fails
(1) If my [wife/husband] shall predecease me or if the foregoing trust shall fail for any other reason then my trustees shall hold my residuary estate and the income thereof upon trust for such of my children [by my [wife/husband] ] as shall survive me and attain the age of [18/25] years and if more than one in equal shares;
(2) provided that if any child of mine [by my [wife/husband]] shall not attain a vested interest under the foregoing trust then any children or remoter descendants of him or her living at or born not more than  years after my death who shall attain the age of [18/25] years or shall be living but under that age at the expiry of  years from my death shall take and if more than one equally through all degrees according to their stocks the share of my residuary estate which such child of mine would have taken if he or she had attained a vested interest and so that no child or remoter descendant of a child of mine shall take whose parent or remoter ancestor attains a vested interest and section 33 of the Wills Act 1837 shall not apply to the foregoing trusts.
Please note there are different precedents in Section 3 Wills for married couples …with ADULT children.