Any beneficiary who is an infant, of unsound mind or otherwise legally incompetent, and also possible unborn or unascertained beneficiaries may be represented in a testamentary trust accounting by the court, or by competent living members of the class to which they do or would belong, or by a guardian ad litem, as the court deems best. If the residence of any beneficiary is unknown, or there is doubt as to the existence of one or more persons as beneficiaries, the court shall make such provision for service of notice and representation on the accounting as it believes proper.