How to claim stimulus checks for kids I didn't claim on taxes?

[xmasbaby0]

Please explain why you are asking that question.  Why did you not claim the children on your tax return?   Were you the custodial parent?   

You do not get a stimulus check for a child you did not claim as a dependent.   Why do you think you do?  In order to get a stimulus check for a dependent child, the child had to be claimed on your 2020 tax return.   If you mean that you are not the custodial parent---then if you and the other parent  had a signed 8332 agreement, then the parent who got the child tax credit could also get the stimulus check for claiming the child.

If that was your situation then you can use the recovery rebate credit on your 2020 tax return to get the 1st and/or 2nd EIP payments for dependent children who were under the age of 17 at the end of 2020.

[DoninGA]

You are not eligible for the Recovery Rebate Credit (stimulus) for dependents you did not claim on your 2020 federal tax return.

[Opus 17]

For 2020:

There were two rounds of stimulus payments in Summer 2020 and Winter 2021, of $500 for each qualifying child dependent.  If you did not get the stimulus check, you could get a rebate if you claimed the children as dependents on your 2020 tax return.  That's the end. If you did not claim them as dependents on your 2020 tax return, you can't get a stimulus or rebate.

For 2021:

You can't get a stimulus check ($1400) for a dependent that you plan to claim in 2021 but did not claim in 2020. You can claim a rebate on your 2021 tax return instead.

Congress has also authorized payment of an advanced child tax credit ($300 per month) for each qualifying dependent.  This is separate from the stimulus and has different rules.  The IRS has said they will set up a web site where you can register a dependent for the payment if you did not claim them in 2020, but that web site is not ready yet.  If you can't get the advance payment or only get a partial payment because of delayed sign-up, you will get the rest of the credit on your tax return.