I have found a couple articles related to how to sign for a minor. It appears that a minor should sign for themselves if they have generated an income and are filing on their own, but that the parent should be responsible for anything related to the parents' return.
https://www.irs.gov/taxtopics/tc301.html
https://turbotax.intuit.com/tax-tools/tax-tips/IRS-Tax-Return/Tax-Filing-Requirements-for-Children/I...
Please let us know if this helps.
