According to the IRS, "Generally, to claim a refund, you must file Form 1040X within three years from the date you filed your original return or within two years from the date you paid the tax, whichever is later" (see this discussion of the 1040X amended return). So DoninGA is correct regarding that 3-year window, and that's frequently the end of the discussion. However, there is that word "generally" in the link above, (as well as the two-years-from-tax-paid rule), and I'm curious about the IRS form to which you refer (exactly which form it is, when you received it, and the other circumstances not mentioned...).
Since there's so much money at stake with the Additional Child Tax Credit (and in addition to the details I don't know about, it's impossible to walk you through any kind of calculation in this venue anyway), I recommend your contacting our Tax Advice Service (here's how) or speaking with a local tax pro (just to make sure you don't leave any money on the table!).
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