PPM is a unit of concentration used to express very low concentrations. It means parts per million, i.e. parts of a contaminant present per million (10˄6) parts of a solution. 0.250 ppm of lead in blood means that 0.250 parts of lead present in one million parts of the child's blood sample.
In units of grams, 1000000g blood sample contains 0.25g lead.
So, 1 g blood sample should contain 0.25/1000000 g of lead = 0.25/(1000000×207.2)g of lead = 1.207 ×10˄-9 moles of lead = 1.207 nanomoles of lead (atomic mass of lead is 207.2).