I think a 2-D horse would be good. You might have to break up the work for this girl, but the end result could be great.
Here is something similar that I did - I made this giraffe for my sister by cutting out a profile of the giraffe, sewing it together, and sewing on legs. This took me a long time, but I sewed on all the spots and had to sew the outline 2 or 3 times to make sure it was closed tight. I think it would be easy to make a much simpler version for your horse idea. And paiting spots would probably be easier and more fun.http://i30.photobucket.com/albums/c308/jgolon/Sewing/CIMG0340.jpg