Our attire is really very simple. White pants/jeans (look for wranglers - lots of feed stores have them around now with the 4-H & FFA fairs going on), long sleeved white shirt, a white scarf (a square folded in half works - for a child probably 25 x 25 in works, if you can't find one would be easy to make). Paddock boots or any sort of riding boot works in brown or black - same with belt. For juniors, a riding helmet. Juniors can use either a poncho or blazer (navy, tan, white, black). It doesn't have to be a riding jacket.
The full attire rules are in the NAPHA rule book.
Any white pants will do. Cotton is best because you will have to bleach them to get them clean after a weekend of showing. My most recent pair came from Kohl's. White jeans. It looks like there are a couple different options for girls on Kohls.com. Kids don't wear ponchos, so you're fine there. Paddock boots are good. Matching black belt is good. Here's a link to the official NAPHA show rule book. http://napha.net/documents/forms/2014_NAPHA_Rules_Final_v5.pdf
Here's the most important part...
Tack & Attire Requirements. Spurs of any type are not permitted in classes designated solely for Juniors. On any horse ridden in Peruvian tack by a Junior in ANY class: retrancas are prohibited, baticola is required, and floron is optional. Juniors 12 years of age and younger are allowed to use the leather covered work stirrups. Leather covered stirrups are permissible for novices or anyone that for safety, comfort, or physical reasons prefers not to use the wooden stirrups (Part II. Sec. 3).
Helmet Requirement. Riders ages 12 & Under, including Lead Line, shall wear helmets, with bicycle-type
helmets being acceptable. Helmets shall be required for all Juniors ages 12 and Under and will be highly
recommended for those 13 – 18 years of age.
I don't know what your experience is with the Peruvian tack, but they are talking about the tailpiece. Juniors must use the crupper, can use the tail cover, cannot use the straps that go around the legs.
Check out Hanna and Naomi Lemke's pages here on Peruvian Horse World. They have some photos from both the "helmet required" age and the "helmet free" part of their Junior careers. I know this is REALLY short notice, but we are actually putting on a beginner show clinic this weekend in the Green Bay, WI area. Sunday, July 27th. Please let me know ASAP if you would be interested in attending. No horse spots left, but we do have some auditor spots available.