A woman must cover her hair and body, apart from her face and hands, from a non-maḥram man. And the obligatory precaution is that she must also cover her body and hair from a non-bāligh boy who understands good and bad if she deems it probable that him looking at the body of a woman would arouse lustful desires.
However, a woman can keep her face and hands up to the wrists uncovered from a non-maḥram man unless she fears he would fall into sin or she has the intention of making him look at something unlawful; in these two cases, covering those areas as well is obligatory for her.