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.
cheers for the comments.....
I dont like using Flash on or off the camera near the horses...as you never know if your going to spook one near you
The conditions look better than they were and with I was using the large Sigma Lens which cuts alot of light out in itself...
Yea, I figured using a flash around most animals would tend to spook them, unless you have them trained to it. You might need to slightly over-expose the overall image, and bring down the rest of the image in post in order to get more light on the faces.
Yeah, I have always been one in the past to believe in the photo should be OOC...but have purchased lightroom and just teaching myself the software and have revisited some photos