My return was rejected because one of my kids failed to select someone else could claim them as a dependent. How do I find out which one?

I would start by asking them-----which of your kids had a job and filed a tax return?    Did they print the returns so you can look at them and see whether the box is checked that they can be claimed?   The rejection for the duplicate SSN should end in a digit--like 01, 02, 03-----look at those and look at the order of the names of your kids on your tax return to find the likely culprit.   When you sort out which child made the VERY common mistake, they will need to amend their return to change how they answered the questions in MY INFO that asked if they could be claimed as someone else's dependent.  Meanwhile, you are blocked from e-filing unless you either get IP PIN's to use, or print, sign and mail your return.   You do not have to wait for the child to amend to pursue either of those remedies.

GET IP pin in order to e-file if your dependent’s SSN was claimed on another return

https://www.irs.gov/newsroom/irs-takes-steps-to-help-prevent-refund-delays-by-accepting-duplicate-dependent-returns-with-an-ip-pin-for-2025-filing-season-taxpayers-encouraged-to-sign-up-soon-for-ip-pin-online-account

https://www.irs.gov/identity-theft-fraud-scams/get-an-identity-protection-pin