The answer to this is to have your client sign the F.8879 offline in wet ink on behalf of the minor child with the notation “By _______________ (fill in the blanks), parent for minor child.”  It would be your responsibility to verify the identity of the child (which you should have already done as part of your due d for CTC, etc. in addition to the parents' whose return(s) you prepared).

It's a waste of time asking Intuit Support technical questions because they are not trained in tax.  They are there to help only with the use of software (theoretically anyway /s) - even then, you will almost always get faster and better answers here.