1. If your child is travelling with grandparents or only one parent, please google and complete an AFFIDAVIT OF PARENTAL CONSENT For Travel Outside The United States Of A Minor Child Without Both Birth Parents Traveling. You will probably never be asked for it, but my son needed it in a train in Paris.

2. Completing application for Nepal visa online (https://online.nepalimmigration.gov.np/tourist-visa). this will shorten the process at Kathmandu immigration. They grant only short stay visa (used to be free for toddlers) at the airport and later you can extend the gratis visa.

There may be additional things you may need. Just sharing my experience (not recent though). Walking (=running for toddler) before boarding usually helps them sleep during flight but every child is different. Kids learn a lot in Nepal with extended family. Best wishes!!