As a father, how do I make sure gifts and duit raya (festive money) are fair between my children in two homes?

As the father, keep a record of your gifts and match their value according to each child's age and needs, not according to which home they live in. Fairness in giving among children is a trust; the Prophet ﷺ said: “Be conscious of Allah and act justly among your children” (Sahih al-Bukhari 2587). Set the same budget for each child before the Raya season so that you are not carried away by feelings at one home. Hand over gifts with the same love, not just the same value. Avoid patching over differences with secret gifts that end up creating a sense of unfairness. If your finances change between two periods, be transparent and consistent. See children in polygyny.