If the other parent already pays child support for children other than yours, that is not an excuse for not supporting your children.
However, if there are prior orders that require the other parent to pay support, and the other parent is paying child support for his or her other children, the court deducts those payments from the his or her gross income before applying the formula to calculate child support for your children.
The place for including this deduction is on the Child Support Guidelines Worksheet.