After writing some different stories that didn't convince me (maybe because I was only focused on the tooth fairy and the zoo), I started thinking more about the diary which is the prop I got out of the blue box.
Well, in that process of creating this story, I was kind of playing with the diary being opened and closed to explain when the story starts and ends. I know it's not an original resource but as it's a short animation it can be great. Then, in act 1, the first character that appears is a kid because the tooth fairy is for kids.
My main problem in writing this story was connecting a diary, a zoo and a story for kids. As in the zoo residents are animals, I remembered a Spanish story. In Spain, instead of a tooth fairy, we have a little mouse called El Ratontito Perez. This little mouse is basically a tooth fairy, I mean it has the same functions. So in my story, the tooth fairy is transformed into a little mouse in order not to be discovered in case she is caught. At this point I came up with the idea of an avatar, a kind of character created by the mouse. All mice have their own avatar.
So, I had to connect the mouse (an animal) with the zoo, which is quite easy. About this I had two ideas:
1) The zoo is the mouse's house.
2) The mouse goes to the zoo to help other animals.