Everyone traveling into Nepal is required to have a full valid passport with at least six months of validity. Along with the passport it is mandatory to have a valid visa for adults and children. You can obtain a multiple entry visa in case you are travelling into another country from Nepal and coming back.
However, this is not important as you can obtain a visa upon arrival easily. You can either apply for your visa in your own country providing that they have a Nepalese embassy or you can obtain one after you arrive in Kathmandu.
Obtaining a visa in your own country will save you the time you will have to spend on queue however you might have to pay more in such cases. So, you are requested to check the prices of the visa if you are monitoring tightly how you are spending for your travel.