That's not necessarily true. Hair can absorb compounds through contact with various different surfaces and follicle tests are notorious for environmental contamination and false positives from things as simple as hair products, which has gotten police officers who handle these cases in trouble in both Australia and the US. Apparently blacks are more susceptible to it (because of course they are), which I guess further conforms Nick's theory that's he's a real nigga.
Seriously though, assuming it's not a false positive, I think there's a reasonable explanation beyond the fact that they were feeding their 9 year old cocaine. First of all, she goes to school, so somebody would have noticed an abrupt shift in behavior if they also recognized the other sighs they were facing neglect. The Rekietas weren't cleaning their house at all and were using regularly, so it's possible that this kid had a chore or an activity that got her contaminated, like she was tasked with buying something with a used bill or handling their clothes or something.
I buy carelessness, arrogance or neglect from the Rekietas, and it's still really fucking back that the kids are in the position at all, but I don't think they're at that level.