The class is closed, so that only those grandchildren in existence can benefit.
Saunders v. Vautier cannot apply as if any grandchild dies under age 25, their entitlement oases to their children. Even if those grandchildren are currently childless, there remains the possibility of them dying under age 25 , but having become a parent in the meantime (applying the rationale from Figg v. Clarke).
Although ss.31/32 Trustee Act 19225 are excluded, the clause has its own scheme for income and capital distributions before age 25.
Clause 1.1.2 brings in reference to the exercise of trustees powers under clause 4.2.1, but this would seem to apply only to income (however, this would need to be reviewed and confirmed).
Clause 1.2 appears to permit trustees to either pay any, or all, of the grandchild’s share to that grandchild, or to use it for their benefit. This would not appear to extend to making any payment to the children of a deceased grandchild before they attain age 25.
On the basis that there are no other provisions in the will which might impact on the effect of clause 1.2, it would seem the trustees could validly terminate the trust by distributing all the grandchildren’s shares now, notwithstanding that some are yet to attain the specified age.
Other contributors have flagged both tax aspects and other points to be considered.