Police on the holiday island of Kos must end the "completely unacceptable" practice of locking up children in adult police cells, UN workers said.
A British mother who is on the island delivering aid to the thousands of migrants sleeping rough said conditions in the cramped cells are "appalling and shocking" with children forced to sleep on urine soaked floors.
Ten children, mainly from Pakistan, who were smuggled on to the island unaccompanied, are being held by police to protect them from dangers including exploitation and trafficking.
But Roberto Mignone, a UN emergency coordinator, said the conditions flout international human rights standards and is urgently working to find a solution.
Kos has become a key gateway to Europe for migrants who have been arriving daily from Turkey in their hundreds.
But unlike other regions receiving an influx of migrants, the island does not have a registration or accommodation centre and a team of four police slowly process the backlog of cases.
It means the estimated 2,500-3,000 migrants sleep rough on the beach or on patches of grass in Kos town, with no access to toilets, just metres from hotels and restaurants.
Rachel Miller, from Nottingham, who cancelled her holiday to deliver aid to the migrants, said: "These children have no air conditioning, they are in their pants sweltering and crying.
"We have been taking food and water to be passed to them. The way the Greek authorities are handling this is horrifying."
Mr Mignone said: "The Greek authorities have good intentions by trying to protect these children but it is completely unacceptable to put them in police cells.
"It is not in line with international human rights standards.
"The children are legally in the care of the Greek authorities. We have offered to house them but because of Greek law a police officer would have to guard the house 24 hours a day and they say they do not have the resources.
"We have arranged for the children to be taken to Greece on Monday as a priority and we will send an interpreter with them to accompany them."
The children will then be taken in to the care of social services.
It comes after the Macedonian government declared a state of emergency as waves of migrants heading north in to the European Union continue to try to rush its border with Greece
Greece has seen an unprecedented arrival of migrants, mainly from Syria, this year, totalling more than 160,000.
Many travel from Turkey to Aegean islands, including Kos, on small dinghy boats in the darkness to gain temporary travel papers allowing them to move on to more wealthy EU countries such as Germany and Sweden.