Gratis Visa:

Children below 10 yrs.
Dependants up to the age of 16 years, holding foreign passports of Nepalese parents or either of their parent being a Nepalese citizen.
Citizen of Nepali origin, after having obtained passport for the first time from a Nepal based foreign mission, until the period of his departure to a foreign country.
Citizens of member countries of South Asian association for Regional Cooperation (SAARC) and the People’ s Republic of china.
Any foreigner, who has lived in Nepal up to fifteen (15) days continuously, re-entering Nepal during the same visa year.
Any foreigner, tourist entering into Nepal for three days or less than that. 

Visa can be obtained either on arrival in Nepal or from Royal Nepalese Embassy or Consulate or other Mission offices abroad. Two passport size photographs required. Indian nationals do not require visa to visit Nepal. However, they require to be in possession of any one of the following documents while travelling between the two countries: Valid national passport and Photo identity card issued by the government of India/any State Government or Union Territory/Administration in India/Identity Cards issued by the Election Commission of India. (Except Tatkal Identity Cards issued by the Ministry of Railways).

Children between 10-18 years age group are allowed to travel by air on the strength of a passport or photo identity card issued by the Principal of their school or college.

Emergency Certificate issued by Embassy of India, Kathmandu to Indian nationals in case of emergent conditions.
Children up to the age of 10 years will not require the above-mentioned documents for traveling between India and Nepal, by air.

For further information please refer:
Dept. of Immigration,Kalikasthan, Kathmandu
Tel: 00-977-1-4223590/223127 Website: http://www.immi.gov.np