Hello Jyeast, welcome to the forum:-), my 6 year old daughter has been riding for 2 years now but nothing like this young girl Wow she can handle a horse

. USM is a simple tool to use (smart sharpen is even easier if you have Adobe CS3) just have a play with the settings as you can see instantly what effect it will have on your pic. i took one of your pics and set the USM at
"amount" = 86 (a Tad too much with hindsight)
"radius" = 0.4
"threshold" =1
The settings are to ones personal tastes and not all settings match all actions, so just have a play until you find your own variant
p.s personally wouldn't set the threshold any higher than 1, play with the other 2 settings.
pps If you have CS3 smart sharpen also has the option of "motion blur", & "lens blur" correction above the "gaussian blur" that USM works on. Kind regards Graham.