Okay, just to play devil's advocate here, the most reasonable explanation for a positive hair follicle test may be that the metabolites are the result of a prescription medication.
In support of this,
- Being prescribed Adderall is a valid defense as to why urinalysis will test positive for methamphetamine (which is not cocaine, but there are similar cases with cocaine).
- 9-year-old had narcolepsy, where stimulants may be prescribed. Unsure, but I think also ADHD?
- If she is the only one prescribed a -caine suffix medication, that would explain why she is the only one that tested positive.
In argument against this,
- Managing drug addicts is effectively the #1 job of the criminal justice system. I find it difficult to believe that the tests do not pull a list of her prescriptions and cross-reference that against her results, especially in the extraordinary case of a 9-year-old testing positive.
- This defense does effectively require the state to lie by omission.
If she was temporarily prescribed a -caine suffix medication like Lidocaine or Novacaine for a routine procedure, that may not show up on her Rx list but would be in her system? But if that's true, would it show up in the quantities demonstrated by the results?

Edit: Dyn is highly qualified to speak on this. If he says there's no chance of false positive, I believe him.