Child support in Illinois is governed by Section 505 of the Illinois Marriage and Dissolution of Marriage Act ("IMDMA"). The amount of child support is based on the number of children born of the marriage, and the paying spouses net income. "Net Income" for child support purposes is a parties gross income, less expenses incurred to produce said income, health insurance payments, social security, mandatory reductions from pay (e.g., union dues, pension contributions, etc.) taxes actually paid, and support obligations from prior marriages/children. However, the new maintenance law added Section 505(a)(3)(g-5), which adds to the list: "Obligations pursuant to a court order for maintenance in the pending proceeding actually paid or payable under Section 504 to the same party to whom child support is to be payable." This means that whatever a party is paying to their spouse for maintenance is taken out of that person's income in calculating child support, which results in a reduction in child support payments.
As an example, let's say Mr. Smith is paying his ex-wife $1,000 per month in maintenance, and has a net income of $5,000 per month, and must child support for his two children. His obligation under statutory child support guidelines would be 28% of $60,000, or $1,400 per month. Under the new calculation, the $1,000 per month in maintenance would be deducted from his income, bringing it to $4,000 per month. The resulting child support payment would be $1,120 per month, a reduction of $280 per month. If the older child is 8 years old, this can represent savings of over $33,000 for Mr. Smith (and more depending on the age of the younger child).
IMDMA Section 510 allows a party to petition for a modification of child support if there is a substantial change in circumstances; a change in the law very well may be such a substantial change. Anyone paying child support and paying spousal support pursuant to an Illinois divorce judgment entered before 2015 should seriously consider contacting a competent divorce attorney for a potential recalculation.