Generally this is taken into account when they decide on the alimony amounts. You just make the alimony payment less because they receive it after tax. The IRS changed this largely due to the fact that the spouse paying the alimony took the deduction, but the spouse receiving alimony did not claim it on their taxes or did not owe taxes on this money.
For child support it is different because everyone pays for child expenses after taxes anyway, so why give them a tax break because they are divorced.
