Following iron therapy in a child with iron deficiency anemia, initial bone marrow response is seen within 48hours. The earliest response seen is a rise in the level of reticulocyte which occur by the 2nd to 3rd day. This is followed by elevation of hemoglobin level. After the correction of the hemoglobin levels, body iron stores are repleted. Ref: Essential Pediatrics, 6th Edition By O P Ghai, Page 302.