Everyone visiting Nepal, with the exception of people from India, are required to have a visa to enter the country. The good news is that they are relatively easy to obtain. You can either get them before you leave home at the local Nepal embassy, or leave it until you arrive in the country. They are available at Kathmandu international airport, or at most of the road borders.
The tourist visas are all multiple entry visas and are valid for three to six months. If you have children under 10 years old you still need to get them a visa, but there is no charge. Make sure that your passport has at least six months of validity remaining and that you have a few passport photos with you.
The visa process at the airport can be slow and tedious depending on how many people have just arrived on the flight requiring visas. It is advisable to get ahead of the game by downloading the visa application form before you arrive. A multiple entry visa valid for 15/30/90 days respectively will cost around US $25/40/100. This can be paid in any major currency at the airport. However, only US dollars are accepted at the land borders.