I'm wondering why you would think that children who have less mercury in their hair would therefore have less mercury in their bodies. If they have, for whatever reason, been unable to properly excrete it, it wouldn't be in their hair, or in their urine. It would likely have crossed the blood-brain barrier, which is a big problem with mercury poisoning.
Because you can check the mercury burden in the body via hair sample. Mercury is not "excreted" into the hair. That's not how the body clears it. It his excreted into urine, feces, and via exhalation.