I have heard that when there are multiple beneficiaries individual designated beneficiary trust should be used to get RMD to stretch for individual’s life span. I have also heard that to do so one should create individual stand alone accumulation trust for each individual beneficiary as per their share at the time of death of Roth IRA owner so each individual beneficiary can use their own age to calculate RMD for life time. Now if beneficiary is 2 years and too young to have children but grandfather want to pass on his grandson’s share to his grandson’s to be born children. Can grandfather assign to be born children of his grandchild as contingent or remainder beneficiaries or merely writing then lineal descendants of his grandchild as his contingent or remainder beneficiaries would work?

I have also heard that instead of doing Stand Alone Accumulation Trust it can be achieved through Testamentary Trust and naming grandchild as beneficiary of that trust and do not have worry about naming his to be born children as beneficiary but trustee could add those name when they are born.

Please, explain which approach will be more appropriate. And also explain how to be born children can be added as beneficiaries.