Every US citizen, regardless of age, must have a passport to travel abroad. Newborn babies, infants, and toddlers all need a passport to leave and enter another country. Just as if an adult was applying for a passport, babies must have Form DS-11 filled out and submitted.

Sorry, I wasn't clear on my question. We are travelling from New Delhi India to Kathmandu. They are Nepali citizens but they don't have their birth certificates or passport yet. What documents do we need to show Nepal immigration for my children to enter Nepal?

The infant (4 months) you're traveling with wasn't born in the US ? If he's born in the US he's a US citizen. Not sure how you're saying 4 months old is a Nepali citizen... unless you brought him from Nepal when he was newborn ?

I see where the confusion is. Sorry for the lack of info. I am in US, but my wife and children are back in Nepal. Both of my kids were born in Nepal. They went to New Delhi couple weeks ago by train. I am flying to New Delhi next month, and flying them to Kathmandu with me. My wife didn't have anything when she took kids to India in train. Now that I want them to fly to Kathmandu with me, I need to know what will they be asking for my kids to enter Nepal. Hope this clears the confusion.