Notarization is evidence, not authority
Families sometimes draw up a 'guardianship agreement,' have it notarized, and assume the matter is settled. Banks, hospitals, and schools generally won't act on it, because the document doesn't do what a court order does: it doesn't suspend anyone's rights, doesn't give anyone standing over accounts, and isn't subject to the court supervision that makes institutions comfortable.
The notarized signature proves who signed. Whether that signature carries any power is a question about the underlying instrument and the court that has to recognize it.
