Can we change child custody arrangement extrajudicially?

It "can" be done that way, but it leaves you with a HUGE risk. A court order for child support has no statute of limitations. At any time, your ex could claim you didn't pay, and come after arrears. Custody orders remain binding until judicially changed, and violations of such orders can bring criminal penalties. Thus, the FAR better approach is to have your new agreement reduced to a written stipulation and order, which you both sign and is then submitted to the court to become a final court order. This protects everyone and eliminates the risk. There are "do-it-yourself" forms available for this type of thing, and having an attorney do it should not cost much.

Well, sort of. You can agree to change custody, and you can make whatever agreements you want as to ongoing support, but there is no way to "add it to the divorce decree." If either of you double-crossed the other (this is usually portrayed as a "misunderstanding" in the later motion filing) the court might enforce the prior orders requiring payment, all the way back to when the money stopped. It is a heck of a lot safer and more certain to put the terms of your agreement in a stipulation and order modifying the existing order. That DOES get "added to the decree" (again, sort of) insofar as it goes in the court file as an order of the court. That each parent has one child is not the end of the analysis for child support under the statutes. The relative income of the parties is also important for that calculation. Full details about both custody and child support calculations are set out on the respective pages of the web site referenced below.