How can one deal with a wealthy father who refuses to buy simple luxuries for his children, such as a cake to celebrate an achievement, on the grounds that they are non-essential luxuries, despite him providing for basic needs?

The father's expenditure on his children is obligatory without disagreement. The Shari'ah did not specify its amount; rather, it referred it to custom. Thus, what is obligatory is what people in their society customarily consider as life's necessities and needs.

Anything beyond the obligatory expenditure is considered an act of Ihsan (benevolence) commanded in Islam. Providing amply for one's family and children is encouraged by religious texts and is one of the greatest keys to winning hearts and increasing love among family members, especially on occasions that call for expressing joy.

As for luxuries, a father might withhold them for the purpose of educating his children or saving for their future interests. These are matters in which the relationship between family members is not limited to rights and duties but also includes affection, tolerance, dutifulness, and good companionship.