Note that child tax credit is "non-refundable" credit -- meaning no matter amount claimed only an amount upto your tax liability can be used . Thus if the total available to you is say $4000 and your tax liability before applying credit is $1800 then ONLY $1800 of the available is used to reduce your tax liability to ZERO, then rest is lost. Depending on your AGI, you may be eligible for a portion of the unused as additional child tax cerdit which is a refundable credit, meaning it increases your refund. Need more details to be specific.
