Billy001

Sharing some additional immigration experience. After landing and heading to an ad-hoc counter thinking it being an information helpdesk, the person indicated he could process the ETV visa for the baby (no one was ahead of us). ETV visa was just a stamp with expiry date.
Our (Indian nationality) immigration had to be done at different counter (normal lines). We gave our passports (which the officer processed) and also gave him babies US passport. Without opening US passport Officer indicated he can't process it. He was not sure of processing hers and then later checking with his colleague indicated nothing needs to be done since visa was already stamped for hers in the passport. We were good to go.

So basically once you get the visa nothing needs to be done (for family travelers with infants having different nationality).
(Would have been ideal if the officer who gave ETV also had processed our immigration. Not sure if it was due to our ETV being processed at a side counter at the entrance instead of main section where there were queues)