Coming from someone who can't spend much money, I have the 75-300mm f/4-5.6 i get fine picture quality with it, albeit not "L" qulaity but still good. However it is not a fast lensby any means. I would think that the 70-300mm f/4-5.6 IS USM would fill your needs nicely. Although not any faster it would be cheaper than an "L" lens and would have better build and image quality than the 75-300mm f/4-5.6 The USM will help track the horse and the IS would help hand holding in lowlight for pictures of your kid. However if you had the money the 70-200mm f/4 is probably your best bet, the f/2.8 even better, probably one of the best lenses I've used