The United Nations warns of a lost generation of Muslim Rohingya children, with half a million in refugee camps in Bangladesh.
They faced dangers including disease and floods and those still in Myanmar lacked access to proper education, UNICEF spokesperson Simon Ingram said.
One year since 700,000 Rohingyas fled a violent crackdown by Myanmar security forces in Rakhine state, the UN Children's Fund (UNICEF) gave a bleak assessment of the outlook for children on either side of the border.
"We are talking about risking the loss or the potential loss of a generation of Rohingya children," Mr Ingram said.
"It isn't just the half a million children or so on the Bangladeshi side of border but it's also those who are still left behind in Rakhine state, whose access to education is patchy at best and highly limited."
The UN estimated 530,000 to 600,000 stateless Rohingyas remained in Rakhine state, including about 360,000 children, he said. The UN has limited access there.
The Rohingya, who regard themselves as native to Rakhine state, are widely considered as interlopers by Myanmar's Buddhist majority and are denied citizenship.
Myanmar's military launched the crackdown in Rakhine a year ago in response to militant attacks. The civilian administration of government leader Aung San Suu Kyi defended what it described as a legitimate counter-insurgency operation.
The administration has said it is ready to accept back refugees. Spaces have been mapped out for the resettlement of people who fled, Ms Suu Kyi said.
Mr Ingram said the prospects for their return to Myanmar soon were bleak, despite a voluntary repatriation agreement signed by the Yangon government and UN agencies in June.
Conditions in Rakhine remained unsafe, he said.
UNICEF said it was expanding education programmes in the camps in Bangladesh, currently for children up to the age of 14, to try to meet the needs of older children.