A trust instrument–whether it is revocable by the creator or irrevocable–names the beneficiaries. A well-drafted instrument will name alternate beneficiaries in case the first named beneficiary dies. This is true regardless of whether the trust instrument was revocable or irrevocable.
For example, the trust instrument may provide something like this:
“The trustee shall divide the trust estate into two equal shares, one share for each of the trustor’s children. If a child dies, the deceased child’s share passes to his or her children; provided, however, that if the deceased child does not have any living children, then to the trustor’s surviving child.