Yes, if you make a mistake when determining the amount of your Child Tax Credit on your 2021 tax return, the IRS will automatically fix that amount and correct your return. According to the IRS FAQ, if there is a mistake with the Child Tax Credit amount on Line 28 of the Form 1040, Form 1040-SR, or Form 1040-NR, the IRS will calculate the correct amount, make the correction, and continue processing the return. Additionally, the IRS will send you a notice explaining any change to the amount of Child Tax Credit that you claimed. This process is in line with the guidance provided in Rev. Proc. 2022-12, which states that the IRS will correct any mathematical or clerical errors on a return and make necessary adjustments to the amount of the Child Tax Credit.
Sources:
Publication 1966
Rev. Proc. 2022-12
§ 24. Child tax credit
