If you didn't receive the additional $500 from the first stimulus check and/or the additional $600 from the second stimulus for your eligible children, you should have claimed the amounts as the Recovery Rebate Credit when you filed your 2020 federal tax return. If you did not, then you would need to amend your return. If you did, you can check your status on your Online IRS account. Here is the link.
There is a third stimulus coming. Here is a TurboTax article with more information.
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