Between January and August this year, over 7,000 children - on average more than 850 every month - arrived in Greece by boat. That's significantly more than in the same period last year, when the number of children arriving was about 5,300. According to the United Nations, even greater numbers are expected to arrive in the coming months.

The children are housed together with their families in reception centers on the islands. There are now an estimated 20,500 refugees and migrants living in these centers - including over 5,000 children - in unhygienic and insecure shelters.

"As the number ... increases, the conditions in these reception centers for these children are becoming worse and more dangerous," says Lucio Melandri, Unicef coordinator for assistance to refugees in Greece. "All refugees in the centers, especially children, must be transferred to the mainland without delay to ensure that they are properly housed and protected."

Authorities unable to cope

Under Greek law, refugees and migrants should not stay longer than 25 days in the reception centers. According to Unicef, some children have been living there for more than a year, despite the efforts of staff and local authorities.  "The Greek authorities and communities have done what they can to help the children ... but they cannnot cope with the large number of people in need," says Melandri.

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The Moria center on the island of Lesbos, which has a capacity of 3,100 people, currently houses almost 9,000 people, including 1,700 children. The reception center in Vathi on Samos, meant to accommodate up to 650 people, has 4,000 refugees and migrants, including 650 children.

Unicef says the children in the camps face health risks and severe psychological stress. Sanitation is inadequate - in some cases, 70 people share a single toilet, leading to flooding with feces.

Melandri, who recently visited Lesbos and Samos, said most of the children there had been through the trauma of war. "Now they live in miserable conditions, and there is no end in sight. Many suffer from emotional stress," he said.

As well as calling for the immediate transfer to the Greek mainland, Unicef says more resettlement opportunities and family reunification processes in the EU must be implemented.