There are some "fun" computer games for horse lovers, but as a professional riding instructor I would not recommend any of them for "teaching" riding skill. There are lots of terrific books out there for all ages that would be much more informational. If she is planning on riding English, THE defacto book is "Hunter Seat Equitation" by George Morris. It is considered the "bible" of the sport. May be a bit technical for younger readers.
A subscription to one of the many horse magazines would be great too...Practical Horseman, Horse and Rider, Equus etc..When I was a kid I adored my mags well before mom ever sprang for my first riding lesson.
It's so great that you are nurturing her love for riding/horses!