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Children are not young adults. The head is proportionally larger and heavier in a child compared with an adult, and the face-brain proportions are different resulting in a higher centre of gravity for a child.
At 3 years of age the head mass is 80% of the adult mass, and at 10 years of age it is approximately 95%. This represents 25% of a child’s bodyweight. If an adult’s head were the same proportions as the child’s then it would weigh 20kg. Click this link for full article on forward facing vs rear facing child seats.