According to Wikipedia:
When the People’s Republic of China was first established in 1949, the State Council (Cabinet) designated a half-day holiday for all primary schools on June 1. This was later made into a full day’s break in 1956 with The Announcement by the State Council to make June 1 Children’s Day a One-Day Holiday.
Schools usually hold activities such as camping trips or free movies on Children’s Day to allow students to have fun, and children of civil servants might also receive small gifts from the government.
The picture is from China Daily, which is doing a commemorative photo gallery of what Children’s Day (also known as 六一国际儿童节, or Six One International Children’s Day) looked like 59 years ago. For history buffs and retrophiles, it’s worth checking out.