that sounds about right to me. Based on what i can see in my neck of the woods. The more affluent kids do seem to be out of sight doing some structured activities..i don't know like maybe piano lessons and extra tuition to truly lock in that upbringing advantage [that's if they haven't been packed off to boarding school]...not to mention probably some supervised and structured sporting activities. there does seem to be a lot more parental involvement in activities, rather than kids being left to a degree to their own devices as we used to be.
They seem to be putting this down to 'wealth' i.e. kids in the London areas spend more time indoors doing structured activities and getting driven around, whereas up here the children spend more time 'on the streets' and riding bicycles. Or what we used to call 'playing outside'.
There are also a lot of other issues conveniently not mentioned in the article which make a difference in the accident rates: