Round 1 and round 2 are claimed on your 2020 tax return. If you did not claim your child as a dependent in 2020, you aren’t entitled to those stimulus payments. If you believe you were entitled to claim your child for 2020, you can file an amended return, but that will trigger an IRS investigation of both you and the grandparents and whoever is found to be wrong will have to repay all tax benefits for the dependent with interest and possibly a penalty.
If you claim the child on your 2021 return, you will receive the round 3 stimulus even if the grandparents were already sent an advance check. The grandparents will not have to repay the advance check if they got one.
