I took by 9 year old niece to WDW last August. I took a note from her parents giving me authority to approve medical treatment if necessary (it also included her health insurance information). She has a passport, so we took that along just in case (she had traveled with her parents the week before our trip and they were given conflicting info regarding minor's ID by the airlines). Some airline employees said everyone must have ID, others said minors didn't need ID.
On our way to WDW, no one at the airport asked for her ID, but in Orlando, when we checked in (at curbside), she was sitting in the car when I handed the skycap our ticket info, my ID and told him I was travelling with a minor....he said "you have the same last name, right?", I told him no, and offered him her passport and had no problem (I don't know if we would have been slowed down if I hadn't had ID for her or not).
I don't think that minors need ID, but if she has ID, it may help you get past a misinformed airline employee quicker (without having to wait for them to call a supervisor and find out they were wrong).
No ID was needed for her a WDW. Have a great trip......my niece keeps offering to go back to WDW with me any time I want.