Here are IRS resources, FAQ, and guidance:
https://www.irs.gov/credits-deductions/2021-child-tax-credit-and-advance-child-tax-credit-payments-f...

https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021

Repayment protection (in Sched 8812):
https://www.irs.gov/instructions/i1040s8#en_US_2021_publink100077789

The third Stimulus payout, which started in Jan 2021, has different eligibility rules than EIP#1 and #2, as to dependents. This payout was also a projection, using 2019 or 2020 tax returns, then reconciled against Actuals on the 2021 tax return. Stimulus info can be found at these links:

https://www.irs.gov/payments/view-your-tax-account

https://www.irs.gov/coronavirus/economic-impact-payment-information-center

https://www.taxpayeradvocate.irs.gov/covid-19-home/3rd-eip-and-2021-rrc/

Good luck.