A male employee reportedly hit the boys, causing critical injuries.
This comes a day after a man killed two before slashing six children at a school in Shanghai on Thursday.
Police in Yulin have detained the male employee and are investigating the case. The boys were in the fifth grade, which generally has children aged between 10-12.
China has seen a number of attacks at primary schools in recent years.
On 14 December 2012, 22 young children were wounded by a man wielding a knife at a primary school in Henan province.
In 2010, a series of knife attacks in schools were also reported.