Thank you for using JustAnswer.
No, a power of attorney does not need to be filed with the courts. A power of attorney is an extrajudicial document that does not need judicial authorization, but rather authorization of the individual that has the authority to delegate the power.
what you will want to power of attorney for the state that the child is living in, and the guardian of the child will fill out that power of attorney, sign it in the presence of a notary, and give it to you. With the power of attorney you will be able to act as the de facto guardian.
Furthermore, you would need to have a specific type of power of attorney, which is a power of attorney over a minor.
You can find such a form here: http://www.uslegalforms.com/wi/WI-P007.htm
Hope that clears things up a bit. If you have any other questions, please let me know. If not, and you have not yet, please rate my answer AND press the "submit" button, if applicable. Please note that I don't get any credit for my answer unless and until you rate it a 3, 4, 5 (good or better). Thank you, XXXXX XXXXX luck to you!