Differences between Dixit versions?
I personally own all of the above apart from Dixit Journey (which I didn't even know existed).

You should know that, Dixit 2 and 3, aren't technically stand-alone games; they're only sets of additional cards.

Dixit Oddysey is nearly the same as original Dixit when it comes to rules. As far as I remember it has all the rules the original had, plus some additions for bigger number of players. It also has a set of pieces for greater number of players (up to 12, I think). The biggest difference are the actual cards - those are entirely new when compared to original.

My recommendation is to buy the Oddysey, as it contains the game pieces helpful* in playing a game and a manual. Then if your niece likes the game you could buy the others for additional cards.

*) As long as you have access to the game rules, you don't really need anything apart from the cards. All the other pieces in the boxes are used to keep track of players score, and to 'vote' for the cards - if you can manage this some other way (easiest and most crude would be pen&amp;paper solution) you could buy only the cards.