Snow White, Cinderella and Aurora most likely never get anything from Santa, so why should they believe in him? Tiana and Mulan are too down to earth. From the ones left, I say Ariel - because she isn't exactly the type who questiones information she has been given, if someone told her "it's a human thing", she would believe it.

Actually think even thought Snow White would possibly never get a present from him she mostly like believe in him judging by her child-ish personality. My next choice would be Ariel.

She believes in god so i think she'll believe in old saint nick.

Shes been in the tower her whole life so she probably would thnk hes real.

Agree with @Swanprise and @Purpledolphin

shes the youngest of all

Or Snow White. Both raw childlike and naive, so I can easily see them believing in Santa.

little girl, naive, believes everything she hears, definitely possible.

or Ariel :)

She can be deceived by Mother Gothel, so why not Santa?