It was not the youngest girl. It was the second youngest girl.
There is a theory that she was dosed. The evidence cited in support of that is that she has some behavioral issues, and Nick believes coke has medicinal value.
Now, personally, I don't know if she was actually dosed, but there are people (including people I respect very much, and not just shit posters here) that believe that.
Alternative theory is she got into it by accident, or was exposed to it by accident.
Ignorant theory (by idiots who don't know what test is testing for) is that some residue just got into her hair (no).
Insane theory is the test results are fake, and the evil gubament planted the drugs (see Robert Barnes)
To my mind, it really doesn't matter if the exposure was intentional or an accident. Your kid shouldn't test positive for fucking coke.