How should I treat my co-wife's children?

Treat them with love and respect; they are your husband's children and bear no blame for the decisions of adults. Do not use the children as 'weapons' or as a source of information about the other household. Respect the role of their own mother and do not try to replace her. The Prophet ﷺ commanded fairness between children; the husband, for his part, must be fair between the children of all his wives in gifts and attention. Kindness to a co-wife's children is often the strongest bridge to harmony in the wider family. See children & polygyny.