If the trustees have an overriding power of appointment then I would use that - avoids having to consider the conditionality of using the statutory powers.
You might also find that if the LT waived his/her interest the result would be that the trustees were holding as bare trustees for the LTs child, if there is no other eligible beneficiary at the point of the waiver. But that would depend on the wording of the interest in remainder. There might simply be a continuing trust for the remaindermen as a group.
Have you thought about CGT? Exit from trust is a disposal, not an uplift on death situation.