Is the zakat threshold calculated for each individual family member separately, or by combining the assets of the husband, wife, and minor children? And is zakat obligatory for me if the total amount of gold and money I own reaches the threshold, even though each category on its own does not reach the threshold?

The view of the majority of scholars is that commingling (al-khultah) does not affect zakat except for livestock (bahimat al-an‘am). If the wealth is commingled, each individual's wealth is calculated separately, and zakat becomes obligatory on it if it reaches the nisab (minimum threshold) and a hawl (lunar year) passes over it. However, if the wealth is not commingled, one person's wealth is not combined with another's to complete the nisab.

If the wealth reaches the nisab and a hawl passes over it, zakat becomes obligatory on it, and it is not a condition for it to exceed the nisab. If gold is jewelry for adornment, there is no zakat on it according to the majority of scholars. If it is not jewelry, it is combined with other wealth, and if it reaches the nisab, the hawl begins, and all of it is subject to zakat upon the completion of the hawl. The amount that must be paid out is a quarter of a tenth (2.5%).