Can you give me example of authorization letter to fetch my child In school?

You may be thinking that this is far more complicated than it has to be. Sometimes companies will have standard forms for this purpose that require little more than a signature. In other cases, and at least for the short term, it would do to write something like:

Dear Mr. Doe, I authorize you to speak to my accountant about my accounts with you in order to straighten out what is needed for preparation of my taxes.

Somebody just wants to make sure that necessary work can be done without violating anyone's privacy.