Theoretically yes, one could use estate planning documents to designate child custody to someone besides a biological parent.
But the other parent certainly could contest guardianship after the death of the Will/Trust-creating parent, even if sole custody was granted in a divorce — regardless of the stated instructions in those documents.
Estate planning documents (such as Living Wills and Last Will and Testament) can provide a suggestion of guardianship for minor or disabled children.
But these documents cannot compel any court to appoint such person(s) as the guardian.
The final decision as to guardianship of minor or disable children will always be left with the appropriate court, who will consider the best interest of the child when making the determination concerning guardianship.
To create estate planning documents with custody designations included, see Standard Legal’s Last Will and Testament legal forms software page or Standard Legal’s Living Trust legal forms software page.