The entire country (Norway) had 0 children traffic deaths in 2019. In total 110 traffic deaths, of which 72% were men, very similar to the past few years. About 2/3rd of deaths were drivers or passengers in a car (the rest motorbike or pedestrian), and of those in cars 1/3rd were not using a seat belt.
So it's likely you could get an immediate decrease in total traffic deaths of close to 20% if you could just force everyone to wear a seatbelt.
So it's likely you could get an immediate decrease in total traffic deaths of close to 20% if you could just force everyone to wear a seatbelt.