Anaestricks

The magic number to remember is 4. 

4ml/kg of packed red blood cells will give you an increase of about 1g/L of haemoglobin. 

For example a 9 kg child with an HB of 5g/L will need 180ml of red cells to reach an HB of 10. (9kg X 4ml X 5g/L = 180ml)

PS - You can use this formula for adults too.