To calculate the desired blood transfusion for paediatric patients, utilising the following formula:

Transfusion volume (ml) = Patient's weight (kg) x [Desired Hb (g/L) – Patient's Hb (g/L)] x Transfusion factor (0.5)

This will provide easy and safe way to transfuse blood to children less than 20Kg in weight

Source:
National Blood Authority Australia. Patient Blood Management Guidelines: Module 6 – Neonatal and Paediatrics. Commonwealth of Australia, Canberra, 2016.