Q. Should I opt out of the monthly payment for child tax credit?
A. It doesn't matter, either way.
Whether you get it in advance or wait til tax time; the child tax credit is a a credit (of the full amount) against your calculated tax. Getting it, in advance, will not effect whether you owe an underpayment penalty.
It's a simple matter of: do you want the money now or later. In your case, a gambling winning could mean taking it now might result in having to pay it back later. But, the underpayment penalty, if any, will be the same, either way.
