Is your daughter on Sandimmune Cylosporine or modified Cyclosporine (like Neoral)? The bioavailability of these drugs are different. So the dosings are different. In the US, it is more common to use the modified version. I think it has been found to be more effective, but don't remember the specifics right now.
Don't get hung up on your daughter's low trough level if she is on the right dosage of the drug. Most children (and some adults like me) are not able to achieve the targeted trough levels because the drug is metabolized too quickly. That's why some people push for looking at the 2 hour cyclosporine levels vs. the trough. The trough is more effective for telling if you are receiving too much of the drug.
For comparison, my trough levels when I was on a normal dose of modified cyclosporine (5mg/kg/day) were between 50 and 75. Increasing the cyclosporine dosage brought more toxicity, but no faster improvement in my counts. It took large amounts of the drug to bring up my trough levels a small amount.
Here is a good paper about this and other Cyclosporine issues. The Cyclosporine in this paper is the modified form, not Sandimmune:
I still encourage you to talk to a doctor on the BMT team now. After doing so, your questions about whether or not to move to BMT will become a lot clearer because you will understand the odds for your particular situation. Your daughter is young. Young kids with a perfect sibling match have excellent survival with BMT. I don't know the statistics for parent donors or for patients with many transfusions. However a BMT doctor will lay all this out for you, and then you will know if you should save this as a last resort or go now.
Wishing you and your daughter the best!