I accidentally claimed 2 of my kids that my ex is supposed to claim. How can I amend that?

Amend your tax return. If you are getting a refund, wait until you get it. If you owe money, you can file the amended once the return is processed. To amend:

• How do I amend my federal tax return for this year?
• I need to amend my state return
• Be sure to include any refund or money paid on your amended return.
• You can pay the IRS directly at  IRS Direct Pay.
• Track your return with  Where’s My Amended Return?

Your ex will have to get an identity protection pin for each child in order to efile.

The other parent may be able to get IP PIN's for the children that will enable him to e-file and claim them---or he can claim them and mail his return instead of e-filing.  He does not have to wait for you to amend to use either option.