I come at this from the perspective of someone whose DH, who is 6 ft 3 and has been cycling on the road for 30 years, was knocked off his bike by an idiot of a motorist, his head hit the windscreen, which shattered, and a big crack appeared in DHs helmet
he was very badly shaken, some minor cuts and grazes - i ordered him to A&E to get checked out!
The Dr there said the helmet probably saved him being severely hurt or even killed - and that was a 13 stone adult in a low impact collision
NO WAY would I let an 8 and 5 yr old cycle unaccompanied. could a reversing motorist even SEE a 5 yo behind them? I am extremely careful even going on and off our drive cos of kids on bikes uop and down the cul de sac
8 is too young to be responsible for an unpredicatable 5 yo who MIGHT do something silly - 8yo cannot be held responsible & if anything happens to their younder sib they will carry guilt forever, let alone the trauma of having witnessed the accident and having no grown up with them to help deal with it