When children are actually endangering themselves or someone else, we should prevent and protect them with our actions - not with words. It is the key to motivating children. When children feel good about themselves, they are more likely to co-operate. We've trained children to disobey because we've given masses of attention to what they are doing wrong. We must reinforce with praise the behaviour we want.
Descriptive praise is informative.
It is not helpful to give a child value judgements. The other dominant idea, borrowed from the work of Rudolf Dreikurs, is that parents should not do for children what children can do for themselves. It teaches parents to teach, rather than to nag. When I started to talk to my son in the prescribed fashion, I felt somewhere between Joyce Grenfell and a robot. But like water on thirsty earth, the praise was absorbed and my son seemed more cheerful and biddable.