two-year-old darcy atkinson was covered in bruises and had traces of stimulant ritalin in his system when he died .
two-year-old darcy atkinson , two , died of brain injuries in december 2012 .
he was covered in bruises and had traces of the stimulant ritalin in his system when he died .
dr wilkins described the injuries the toddler sustained as ` tram-line marks ' .