How do I attach proof of placement for foster kids?

[xmasbaby0]

If you are e-filing you cannot attach anything.   Why do you think you need to include proof?   Is someone else trying to claim them?

If you can claim the children on your tax return and someone else files first and claims them, you will not be able to e-file.   You will have to print, sign and mail your tax return.  The IRS will process your return that claims the children.    Then the IRS will sort it out.   It might take up to a year but letters will be sent and then you will show the proof that you could claim the children.  If they are wrongfully claimed by someone else, that person will face repayment of their refund and penalties.

[Critter-3]

You don't  ... that is only needed if the IRS wants proof later.