Cal. Prob. Code § 15405

For the purposes of Sections 15403 and 15404, the consent of a beneficiary who lacks legal capacity, including a minor, or who is an unascertained or unborn beneficiary may be given in proceedings before the court by a guardian ad litem, if it would be appropriate to do so. In determining whether to give consent, the guardian ad litem may rely on general family benefit accruing to living members of the beneficiary’s family as a basis for approving a modification or termination of the trust.