I am all for free range kids, our kids have many more freedoms than almost all their friends (my oldest is 8).
BUT...at disney, they would stay with me. And here is the only reason why, we are on vacation. We go on vacation as a family, I want to spend my time with them. Granted, I might very well change my mind when my kids are a little older,but I see no need for them to be off on their own, when we are on vacation together.
But, honestly, if you trust your 12 year old, and her friend, I think they would be fine. Set them up with a cell phone to contact you if needed and enjoy!