Stayed at ASMo for 10 nights in 2005, gave a good tip every day and got one towel animal (a bird) and one day the room was filthy and we didn't even get soap or shampoo. Stayed at Pop in 2006 for 14 nights and got no towel animals. I also have a child who always has a stuffed animal and toys with him so they knew there was a child in our room. I don't think there really is a way to make it more likely to get towel animals with tips, children, notes etc. My opinion, is it's luck of the draw. With all that being said getting or not getting a towel animal will not mean you will have any less or more fun at Walt Disney World. It is all in what you make of your vacation and enjoying the possibility of a magical moment. I can't say I had any less fun on our trip without towel animals than the one with but it is always nice to hold out hope for any kind of magical moment.
