I think the towel animals depends on the mousekeeper rather than the tips (although it probably does help).
We tip either 4 or 5 a day for our family of 4. We have always gotten lucky with extra lotions, soaps, and lots of towel animals. One day, the mousekeeper took our stuffed animals, put them on the bed and surrounded them by some chips and canned soda we had in the room. They were having a picnic on the bed! It was too cute. Some of the mousekeepers are very original, and IMO they are always grateful for the tips and extremely friendly. I do however, believe that tipping is by choice and not mandatory.
The kids decorate plain envelopes with markers, stickers, etc to help. The kids also left a note of gratitude every day.
Have a blast!