My wife and I received the Economic Impact Payment in March 2021 for ourselves and our two children who were full time students in 2020, but were no dependents in 2021 - they both graduated in 2020. What happens now?

[xmasbaby0]

Nothing "happens."   You do not have to pay back the EIP payments you already received.   You do have to enter your amount correctly as shown on your letter 6475 to show that you received $1400 x 4= $5600 because you are not eligible for any more.   If the kids are not being claimed as dependents and they are filing tax returns Congress left a loophole that allows them to get the $1400 on their 2021 tax returns.

Enter the information in Federal Review for the recovery rebate credit.