The adoptive parents should have filed a Form W 7A with the IRS and obtained an ATIN which would have allowed them to claim the child as a dependent along with any related tax credits.
To be honest until I did some reading, I didn't know ATINs even existed.
Just wanted to point out that S corp distributions need to be proportional to the shareholders' ownership percentages. That's why ATX allocated the distributions the way it did. When this happens with one of my clients, I have them make a corrective distribution as soon as possible.
Note that the PATH act of 2015 changed the law so that one cannot use an SSN or ITIN obtained after the filing deadline to claim credits such as EITC, CTC or ODC, so there may not be any benefit to filing an amended return.
From Schedule 8812 instructions and Form 1040 instructions (for EIC)