Kid traveling with my parents - Letter?

[kingno1]

Hello,
My kid (8 months) who is US Citizen with PIO card is traveling to India along with my parents.
What kind of documents do I need to send along with her apart from her
1) U.S. Passport
2) PIO Card
3) Birth Certificate
4) Medical Vaccination document
I heard about some permission letter or something like that. Where can I get that format and what needs to be done for that paper?
Thanks,