Does your gym have a sportswear store associate? I would go try on the shorts, personally. They're too expensive not to make sure you've got the right size. The problem is that you want the shorts to be pretty tight, or else they'll cause problems while she's trying to work out. My daughter was on the borderline for the shorts, so I went the next size up, but then she said they were making her slip on the bars. I figure eventually she'll grow into them!
And oh, by the way, hers did seem to shrink *slightly* even though they never went in the dryer.
Our gym has a "leo swap" a couple of times a year where you can buy/sell/trade outgrown gym clothes. If your gym doesn't have that, you could try organizing one.