The IRS wants their money. The don't offer payment plans. They are dealing with millions of taxpayers.
That being said, if you fall behind, you can contact them and work out a 'plan' but it won't be any advantage over paying now. The penalties and interest are very high for not paying, and mostly they will just knock some of that off.
They do take credit cards... I wouldn't start racking up those 20% interest charges though.
There are tax reduction companies out there, mostly ex IRS agents, who for a fee will negotiate on your behalf. Be careful who you pay.

As for child support, you go back to court if your income has been reduced and ask the court if they will lower it. Pleading an inability to budget and control spending, forget it.