Myanmar has rejected the group's report saying HRW "based its report on the baseless accusations and exaggerated lies of insurgent groups on the border."
Allegations that both the government and ethnic rebel fighters forcibly recruit child soldiers are long-standing and have been highlighted before by the UN.
But the latest charges come as Myanmar faces widespread international criticism over its crackdown on anti-government protests last month in which human rights groups say hundreds are thought to have died.
"Until the generals' military hardware is crumbled, they won't listen to anyone"
Oomlwin, Yangon, Myanmar
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In its report Human Rights Watch said recruiters routinely falsify enlistment records to list children as 18, the minimum legal age for service.
The group cited the case of a boy who said he was forcibly recruited at age 11, though he was only 1.3 metres tall, and weighed less than 31kg.
It said child recruits were "sometimes forced to participate in human rights abuses, such as burning villages and using civilians for forced labour.
"Those who attempt to escape or desert are beaten, forcibly re-recruited, or imprisoned".
Rejecting the allegations Ye Htut, deputy director-general of Myanmar's information ministry, said the allegations were "another example of biased reporting" by the New York-based rights group.
In an email response to the Associated Press, he said Myanmar's armed forces have had regulations since 1973 forbidding forced inscription and the recruitment of minors.
Ye Htut said: "If the authorities find out that a recruit was recruited against his will or he is under 18 years, the responsible personnel will be tried according to the military law."