Unless they are
a US citizen,
US national, or
US resident alien,
they're ineligible for the credit.
This, from the IRS:
Credit for Other Dependents
Dependents who can’t be claimed for the Child Tax Credit may still qualify you for the Credit for Other Dependents. This is a non-refundable tax credit of up to $500 per qualifying person. The qualifying dependent must be a U.S. citizen, U.S. national, or U.S. resident alien.