Why can kids in Japan get money during New Year?
It is called ‘otoshidama’（おとしだま・お年玉）, which parents usually give to their children. The money can be considered as power from a god.
In Japan, it is believed that a god visits them during New Year and leaves power (or spirit) into a special rice cake displayed in the house so that they can live well and have a good year with it.
The main person in the family, father for example, is allowed to share it with other people in the family by letting them eat the rice cake.
This is the origin of おとしだま. Parents started to share ‘the god power’ by giving money to their children in 1960s. Money is as important, influential and strong as the god power. That could be why.
The money is enveloped and should be given formally. You don’t have to envelope a lot of money. Grade 1 kid may receive 100 yen. 200 yen is for Grade 2, 300 yen is for Grade 3 and so on.