(I.R.C. Sec. 6166 election not available to extend time for payment of generation-skipping transfer tax imposed on death of child as a result of the termination of the child's interest in the trust; I.R.C. Sec. 6166 election applies only to direct skips resulting at the death of an individual; thus, generation-skipping tax imposed on taxable termination is not eligible to be paid in installments).