why can't you bring a practice board or maybe a small keyboard with you?
Personally if I was relaxing in one of the resort lobbies I wouldn't want to listen to someone's kid practicing on the piano. I am sure she is good, but practicing is boring repiticion, not something I would want to hear.
I am sure Disney feels the same way and that is why they lock the pianos, they don't want someone just coming along and banging on keys when it is costly to tune a piano.
I am not meaning this as a slam to your daughter's abilitys, but you would be overstepping boundaries. It might be "cute" to see a little one play but maybe 2 minutes max would be enough "cute" for me.
Just to clarify in our case, this was not a "practice" session at all. With permission, It was a performance of her recital piece, The Black Swan, which is certainly more than a "Row, Row, Row Your Boat" song. She played the one song, thanked the pianist and then she sat back down to hear the rest of his songs.