HRW said it came across dozens of armed soldiers who looked younger than 18 since protests erupted in February in opposition to Yemeni President Ali Abdullah Saleh.
The rights group said the child soldiers, some who gave their age as 14, were members of forces loyal to breakaway Gen. Ali Mohsen.
"The Yemeni government has for too long placed children at grave risk by deploying child soldiers on the field of battle," Joe Stork, deputy Middle East director at Human Rights Watch, said in a statement. "President Saleh's opponents should not perpetuate the problem by using children for security on the field of protest."
Fighting in Sanaa, Yemen's capital, killed five when gunfire broke out between rebels loyal to Mohsen and pro-government forces.
Yemeni media reports this week said Mohsen was training members of the youth movement in the country to attack infrastructure and the government ordered extra security to the international airport in Sanaa as a precaution.
"The United States and other governments should call for an immediate end to the use of children as soldiers or in other security forces, whether for the Yemeni government or the opposition," the rights group said.