Tarot is a game for all ages...she is sure to enjoy the imagery - and it will enhance her imagination. If she shows interest in "how it works", then teach her - otherwise - just let her play with a deck (her own, preferably - or you might find yourself missing cards!). You can also use tarot to teach her other things: how to count, how to recognise certain animals, trades, activities, cultures; you can also start on simple symbolic values: e.g. 8 of Pentacles is like a task she does that takes her time & that she is still learning (is she learning to ride a bicycle, for instance? You can link that card to the concentration & practice she puts into that). Later, you can give her an early grounding on systems.
I started with my nephew & niece when they were a bit older - my nephew first got interested in my cards when he was 7. Following my discussion of what kind of deck I could give him (by that time he had just turned 8), a lovely AT'er gifted him with the Olympus Tarot - which was a great choice, because he loves mythology. So he is learning both Tarot and mythology at the same time
Later, because he had fallen in love with the Tarot of Prague, another generous AT'er gave me one of those for him - he loves it: the art, the characters, the history - everything! My niece, who loves animals, received the Baroque Bohemian Cats, and is enchanted with them - both because of the cats and the beautiful costumes & settings.
But I can imagine all sorts of explorations younger children can do with tarot, or thanks to it, whatever their tastes.