How to increase hemoglobin of a 1st grader child from 9g/dl to normal level of 12g/dl?

Usually iron. The most common anemia in children is iron deficiency, so usually giving enough iron can correct the anemia over time. I would talk with your pediatrician to find out the proper dosage and for how long to give and have your childs blood checked again.
Depends on cause. For most this is due to iron deficiency and a diet rich in iron containing foods or an over the counter iron supplement can help. If the kid has a genetic defect in blood production called alpha thalassemia, no amount of added iron is likely to "normalize" the level, because this level is often normal for those kids,