I am truly sorry for your loss. I hope that you find grace and comfort as time continues to march forward.
I'm not savvy regarding how this works, but I offer you an idea or two. First, you can see how it is that photos are chosen for the Magic, Memories and You show. I don't know if this is even possible, but inquire as to whether or not they would be willing to place within the show a photo of you and your wife holding an 8x10 photo of your daughter within some location at the Magic Kingdom. You could then photograph and videotape the evening show.
I realize that you are probably looking for something permanent, but I'm not aware of anything of that nature that is available at this time. Another thought is to perhaps take a stuffed animal or some other prized possession of hers, and photograph it at specific locations within the Park.
My ideas are a little lame, but it is only because I wish that I could help you further. I hope that this trip becomes more sweet than bitter for you and you wife.