can someone claim a dependent living in mexico like a nephew or a grandchild with an ITIN?
Not for CTC or ODC. But it's still possible for certain benefits such as medical expense deduction if that's what you're looking for.
Why would you want to ??
...................... because it's common to send money home to support relatives and claiming additional dependents makes the tax go down
Skylane What credits do you think claiming will lower their tax, by claiming a nephew or a grandchild as a dependent