What I'm saying is that I think there is little to no credit report, if any at all, for a minor that DocuSign will be able to pull for verification.  And I speculate that it is unlikely that DocuSign will be able to differentiate an adult not having sufficient credit history vs a minor who just isn't on the record at all, considered they don't have all of the demographic variables of the individual concerned.

E-signature can still be used but you may just need to use an alternative authentication method other than credit report-based KBA.  And you should ensure that the notation “By _______________ (fill in the blanks), parent for minor child" can be added to the signature line.

I may have missed it but where in Pub. 1345 does it mention children under and over the age of 16?

Instead, I'd rely on Treas. Regs. §§1.6061-1 and 1.6012-1(a)(4) for how returns should be filed and signed by or on behalf of minors.

Since §6061(a) authorizes the Treasury Secretary to prescribe regulations which, in turn, refer to instructions issued with respect to such returns, statements, or other documents, I'd take the IRS' instructions as golden.

Given the IRS' instructions for signing minors' returns haven't changed since the beginning of e-filing and e-signature, I would apply the exact same instructions to F.8879.