Road safety experiments and road safety data only show show a significant increase in child safety when the use of booster seats is compared with a child not wearing a seat belt. The comparison between children wearing a normal seat belt, and those using a booster seat only shows a small increase in safety. Also a badly fitted child seat is potentially more dangerous, than no child seat at all, and they are generally complicated and difficult to fit.
The requirement for all children to use child booster seats does however increase the number of car on the road by making it much more difficult for parents to share lifts with other parents, it also reduces the ability for other adults, such as grandparents to help with the transportation of children. All of these extra journeys will both increase direct danger to children, especially around the crowded entrances to schools. It also pointlessly increases the congestion on the roads for all other road users.
Instead of a requirement for child booster seats the law should be changed to make all car manufacturers fit child friendly safety belts. That is a technology that could actually make a difference to child safety.