I used to have to buy boys pants/jeans at Sears when I was a string bean young girl. The boys pants are narrower. My son is now 7 and a string bean himself. We find "slims" and sometimes "extra-slims" at Sears and sometimes Marshalls stores. I also sometimes just take a little tuck in each side of the waist band and have him wear sweaters/shirts that hang over the pants so you don't notice the tuck.
It's really hard, I know - especially for an 11-year old girl who is becoming more aware of what is fashionable. You might also try scoping out some tailors in your area. Sometimes they can work miracles on a few pairs of pants for around $10-$15 each (buy pants to fit the length, then have tailor take them in at waist/hips). Good luck!