Been there, done that, still doing it with junior sizes!
First off, and this was the simplest solution by far: buy the size thats larger and fits, (and this is the perfect time of year for this!) and BUY CAPRIS. Then you don't have to hem it, and they're the right size. DONE! (This worked particularly well for us when we were stationed overseas.)
Second, I know Penney's can be expensive, but in my experience, (combined 26 years experience here!) they are the best deal: they last longest, have survived two kids in uniforms for three years, and 1 of my girls is just really rough on everything she owns. The Arizona brand is best, and they also sell Izod as well. Most of Arizona has adjustable waistbands in the girls plus size as I recall, but its been a while.
And here is where being thrifty comes in: I had to buy all of my kids' clothes new theis year, as they are both in uniforms, and either someone went down three sizes (YAY for her!) or the clothes that we did have were just plain worn out after my kids' being in the same size for three years. The pants that made it for three years of wear? JCP Arizona brand or IZOD. Some of them I had to replace only because they didn't fit anymore!
Right now, JCP is having an additional 40-50% off of already clearanced items. ALSO, there is a coupon worth $10-15 off your purchase of $50-75 or more. Here's the link: http://f.chtah.com/i/2/95827765/welcomemain.html?
Usually they will let you use both if you ring them on separate receipts.
Also, join their email list and you will get coupons in the mail and via email. Totally worth the extra clutter in the mailbox!
Otherwise the circo brand from Target was pretty good. Also, the Faded Glory jeans from Wal-Mart tend to have the expandable waistbands as well. Just get the capris from the older girls section. If she is already developing curves, some of the junior clothes will work too.
Like I said, I have years of experience here. The other thing you will just have to do is learn how to hem pants. It will save you lots of money in the long run. Even now, one of my girls is in typical sizes, but we're short, and I still have to hem even the short cuts of pants.
Sorry for the novel, but I just felt I had a lot to offer here.
Also, make sure they do test her hormones, and make them keep an eye on her. The medical community is just now aware of metabolic syndrome; how it develops even in infants/toddlers and the damage it can do even at young ages.
You are her best and only advocate: no one will care about her health like you do, and the medical community may act like they care, but you are Mama, and you know what is best for her. The docs will screw around and let your kid have tons of health problems before they'll do anything. And then they will blame everything on you anyway, just because you're Mom! (Don't EVEN get my started on this!!!)
Best of luck!