Your concern for your daughter is laudable.
If Differin is the only medication she has tried, I would certainly try other and more medications before jumping on Yasmin ( I prefer Yaz by the way). If she was getting Differin 0.1 this could be increased to the Differin gel 0.3. I believe that Tazarac iis stronger and she could try this. A benzoyl Peroxide should be added as well as an antibiotic. Certainly, an antibiotic such as Adoxa or Doryx ( Doxycycline) or Soladyn ( Minocycline) can be used. Diet in the way of a low fat or South Beach Diet is recommended. Vitamins including zinc and Vit C and E. There is a new topical medication out called Aczole ( topical Dapsone). Finally, she might benefit from deep pore cleansing and extractions, photodynamic therapy or blue light IPL, and laser with the Isolaz. All these should be attempted before placing an 11 year old on birth control.
I would consult again with your dermatologist. If she has any problem with facial hair you might point this out and suggest a hormonal work up. This might be especially valuable in a girl at this age with what seems to be fairly substantial acne.
I would be extremely reluctant to place a girl of this age on birth control pills. In fact in my 28 years of private practice I do not believe I have ever placed a female on oral contraceptives below the age of 15.
To anwer your last question, we often do prescribe birth control pills for acne. I prefer Yaz, Drosperinone is used as the progestational agent. This has anti-androgen properties equivalent to Spirolactone 25 mg., an androgen antagonist. This means that, instead of a progestational agent which usually flares up acne (the increase in progesterone in the week before the period is why acne flares so often at that time), you have an anti-acne agent working for you. Still, 11 WAY too young for birth control pills.
