Trust Registration Service/minor beneficiaries

My client wishes to make substitute provision for potential grandchildren in the event that any of her children (both currently under 18) predecease her.

Each daughter’s share would be worth £162,500. There is only one executor / trustee (testator’s brother). She has no one else to appoint as substitute.

If one daughter predeceases leaving very young children, and if the brother/trustee dies during the trust administration period or before the testator, it is not cost-effective to pay a professional trustee to administer the trust for 18 years.

This is a common scenario and will probably dissuade many testators from making provision for their grandchildren, particularly as all estates which are not wound up within two years of death now have to be registered with TRS.

How does one make provision for grandchildren in this scenario? Or is it just not possible/viable?

Thank you.