First One:
The International Children's Day had its origin in Turkey in 1920 (April 23, 1920) and later in the World Conference in Geneva, Switzerland,which 54 representatives from different countries gathered together to convene the first "World Conference for the Wellbeing of Children" in 1925.

It is not clear as to why June 1 was chosen as the International Children's Day: one theory has it that the Chinese consul-general in San Francisco (USA) gathered a number of Chinese orphans to celebrate the Dragon Boat Festival in 1925, which happened to be on June 1 that year, and also coincided with the conference in Geneva.

Second One:

Children, also known as "June 1 International Children's Day," the annual June 1, the world of children's festivals. November 1949, the Women's International Democratic Federation council meeting held in Moscow. China and other countries and representatives of countries and anger exposed the imperialist reactionaries kill and poison children. In order to protect the world's children to survival, health and education, to improve the lives of children. The meeting decided in the annual June 1 International Children's Day.

The International Children's Day (ICD) is celebrated in numerous countries, usually (but not always) on June 1 each year.

Source: globaltimes