Deed of Appointment and Deed of Variation - 18 year old and age contingency of 25

I am acting for an estate where the deceased left his two children a £500k legacy (gifted to them equally) contingent on attaining 25. Due to other gifts in the Will, it will cause IHT to be paid, even though deceased has a surviving spouse.

One child is only 14. The other is 18 this month and wants to vary the Will to gift her legacy to her mum with the view that when she dies, she will benefit from her mum’s estate.

We wish to vary the Will, but I can’t see how because where the 18 year old needs to acquire the age of 25, it will be too late for the IHT saving due to the 2 year window. The youngest can’t agree to the DOV as he is only 14.

I am wondering - Can we not appoint £250k to the eldest under s.32 Trustee Act, and do a Deed of Variation too? Would there be any issues with this?

Many thanks