You have the making of a product liability bonanza/nightmare here, so there are forces at work distorting the science.
I have read that some children have less of an ability than others to eliminate mercury from their bodies. It tends to collect in a certain part of their brain. Do I recall correctly?
I looked on the Internet and came up with these:
http://www.mercola.com/2005/feb/26/mercury_lies.htm
http://www.stepstoperfecthealth.co.uk/articles/mercury_factsheet_heavy_metal_poisoning.htm
http://www.stepstoperfecthealth.co.uk/articles/mercury_factsheet_heavy_metal_poisoning.htm