Have you tried written or pictographic communication? She should be able to understand drawings/pictures. Maybe take "Laney" to the hospital, have them scan her (claim brain damage or something because she had a fall). They may notice physiological differences from a human child. Or even better, they may be able to cure her (though it's unlikely) and you can trade the healthy creature back for your daughter. Figure out some sort of trade, find something that would be valuable to them. Try different things; gold, precious stones, jewelry, enchanted objects, etc. Trading would likely be a lot smarter than trying to threaten them, they don't care about the child they left. Best of luck to you, please update us as soon as you can.