For children with fever and petechiae/purpura consider using the Barts Health NHS Trust guideline for workup. This guideline performed well head to head vs other guidelines in Petechiae in Children (PiC) study.
Purpura
Abnormal platelet count and/or coagulation
Seabather's eruption
Septicemia
Black hairy tongue
Idiopathic thrombocytopenic purpura 
Paronychia
Hemolytic uremic syndrome