Is it permissible to marry a woman who divorced her husband, who practiced sorcery? And is there any harm from her ex-husband to the (new) husband or his children?

A woman's divorce (khul') from her husband does not prevent the validity of her marriage to another man once her 'iddah (definition) (waiting period) has ended, whether the khul' was by mutual agreement or by a judge's ruling. You are permitted to marry her, provided there is a guardian (wali), and this is the view of the majority of scholars. There is no need to fear her former husband who is accused of sorcery; all matters are in the hands of Allah, and protection from sorcery is achieved by relying on Allah and by consistently performing the prescribed remembrances (adhkar) and ruqyah (definition) (incantations).