you will need to modify the court order for each child via each state.
go to each states web site and look for links that will take you to the child support guidelines, most forms are printable via your home now days.
fill out each form and mail them in.
keep in mind the first child will always have priority over the other child/children. however courts will take into consideration the fact that you pay child support for 2 different children.
you have to prove why you want the decrease.
good luck.