Assuming that you would only consider opening a UTMA for a minor child, those who can certify the TIN are:
* the child,
* a parent or legal guardian, or
* the custodian of the UTMA.
The UTMA is a bona fide legal structure where all the relevant questions are answered clearly in the statute; e.g. what happens if the custodian dies, what happens if the minor dies, when is the minor entitled to delivery, etc..
All the other junk accounts; e.g. "Minor-by," accounts are an exercise in pure foolishness where indecision and big customer relations problems hide behind small amounts of money.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.