I think you can safely assume Americans don't let their small children drive after taking melatonin.
Seriously though, do people here really believe melatonin has such serious side effects that it shouldn't be available? That it does harm to hundreds of thousands - or even millions - of people all over the world?
Every drug, vitamin, supplement, food, and beverage you take has side effects. Some are mild, some are more severe. Not everyone taking a certain item will get any or even some of the side effects.
At this point in time, the continued prescription only stance on melatonin in the UK seems very strange given that it is used with no issue at all by so many millions of people elsewhere. It's as atrange as if the British medical establishment insisted on prescriptions for aspirin.