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IRS Publication 17, page 8, states:
If a dependent child must file an income tax return but cannot file due to age or any other reason, then a parent, guardian, or other legally responsible person must file it for the child.
If the child cannot sign the return, the parent or guardian must sign the child's name followed by the words “By (your signature), parent for minor child.”
Page 15 states:
Court-appointed conservator, guardian, or other fiduciary. If you are a court-appointed conservator, guardian, or other fiduciary for a mentally or physically incompetent individual who has to file a tax return, sign your name for the individual. File Form 56.
IRS form 56 Instructions here.
[Edited 03/04/25 | 7:00 pm PST]
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