Who filed my child on their taxes

First its not a good idea to piggyback your question on someone else's. This can lead to confusion. You should start a new thread. So I've moved your question to its own thread.

We cannot tell you, only the IRS can. There are dozens of threads on this issue. E-File has changed things on this issue. When you E-File, the IRS automatically checks the SSNs listed on the return against previously filed returns. If there is a match, then the E-File is rejected. You can still file by mail. Then the IRS will investigate and allow the correct parent to claim the child.