Now for the kids ... amending is possible but not until after the IRS has fully processed the original return AND your refund is in your hand.
In the meantime you need to figure out a few things ... like who is the custodial parent and are they allowing the non custodial parent to claim the kids for the CTC. By filing for EIC only on the kids that indicates you were the custodial parent who was going to allow the non custodial parent to claim the kids for the CTC. But the EX messed up and tried to claim them for the EIC also hence the rejection.
Here are the rules :
If both parents live together with the child for the same amount of time during the tax year then both are the custodial parent ... in that case only ONE of the parents can claim the child ( for all child related benefits) and the other doesn't mention the child at all on their return.
However if the parents do NOT live together all year ...
There is no such thing in the Federal tax law as 50/50, split, or joint custody. The IRS only recognizes physical custody (which parent the child lived with the greater part, but over half, of the tax year. That parent is the custodial parent; the other parent is the noncustodial parent.)
Who can claim the exemption and credits depends on who is the custodial parent. (By the IRS definition of custodial parent for tax purposes - this is not the same as the legal custody that a court might grant.).
The test that the IRS uses to determine the custodial parent is where the child lived for more than 1/2 (or greater part) of the year. The IRS will go so far as to require counting the nights spend in each household - that person is the custodial parent for tax purposes (if exactly equal and more than 183 days - The custodial parent is the parent with the highest AGI, if less than 183 days then neither parent has custody so the child cannot be claimed by either parent). And yes they are that picky.
See Custodial parent and noncustodial parent under the residency test in Pub 17
https://www.irs.gov/publications/p17#en_US_2017_publink1000170899
Only the Custodial parent can claim: (Child would be listed as non-dependent EIC & CC only)
-Head of Household
-Earned Income Credit
-Child Care Credit
The non custodial parent can only claim: (Child would be listed as dependent)
-The Exemption
-The Child Tax Credit
-The Education Credit
But only if specifically specified in a pre-2009 divorce decree, separation agreement or the custodial spouse releases the exemption with a signed 8332 form - after 2009 the IRS only accepts a signed 8332 form that must be attached to the non-custodial parents tax return.
