Your grand niece is very cute with that pony.
One comment, not sure if you're able to, but a fill-flash (preferably a 430EX or 580EX class, or even off camera) would help greatly for riders faces.
Also you're at ISO 400 @/f13, any particular reason you are stopping down so far instead of f/7.1 or f/8 or so? 1/500 is good, but you probably could go faster with ISO100 @f/7.1, and you'd probably get slightly sharper images, since on a crop you tend to (depending on pixel counts) start hitting diffraction limits quicker than full frame.