Idomeni - At a migrant camp on the Greece-Macedonia border, children queue barefoot in the mud and rain for a single sandwich, others wade about in flooded fields.Days of heavy rain have turned Greece's Idomeni border camp into a foul-smelling bog, exposing migrant children to raw sewage, noxious fumes and bitter cold, with aid workers describing conditions as "critical".More than 14 000 mainly Syrian and Iraqi refugees including many children are camped out at the squalid camp where they have been stranded by Skopje's decision to close the frontier. Macedonia has not let anyone enter since Monday.In an effort to keep warm and dry, thousands of migrants are burning whatever they can lay their hands on."People are trying to stay warm so they are burning plastic, they are burning their clothes," Imad Aoun of charity Save the Children said.'Pretty critical' "A lot of children are inhaling this toxic gas. There have been a lot of cases reported of children with respiratory conditions just because they have been here for so long," he said.Aoun described the conditions as "pretty critical", adding that they were some of the "worst that we've seen"."The bathrooms are flooded, there is sewage water everywhere, you see a lot of children as well playing in the sewage water," he said.The tent settlement - where many of the migrants have now been for weeks - was originally set up for just 2 500 people. With around 14 000 in the camp now, a further 6 000 have been forced to sleep in muddy fields and ditches."This area is a sinkhole, it draws in all the rainfall of the region," a government source in Athens said.The refugee build-up began in mid-February after a string of countries including Macedonia started limiting the number of people allowed to cross their territory en route to wealthier countries such as Germany and Scandinavia.Late Tuesday, Slovenia and Croatia both said no migrants would be allowed to cross, with Serbia indicating it would follow suit.Long queues The Greek government has now stationed medical teams in the area to provide primary healthcare.There are already long queues outside the two mobile units, with hundreds of parents holding crying and coughing children."Dozens of children are severely ill, suffering from high fevers, coughs and vomiting," executive director of the Doctors of the World charity, Leigh Daynes, said after a visit to the border."Even before [Tuesday's] border closure our medics saw many children suffering from diarrhoea, hypothermia, croup and severe nappy rash," Daynes said in a statement.Dozens of people have also suffered panic attacks, fainting and hysteria after realising that they could no longer continue their journey, he added.The most serious cases have been taken to hospital in Kilkis, the nearest Greek town around 50km from Idomeni, which is already overwhelmed."We currently have 34 children suffering from respiratory ailments, viral infections and gastroenteritis," said Vassilis Triantafyllidis, head of the local medics' union.Several pregnant migrants have given birth at the hospital, with others miscarrying due to poor conditions at the camp, he said.'It was hell' Government officials are urging refugees to abandon Idomeni and seek shelter in less crowded relocation camps elsewhere in Greece."We are telling them that 20km away there is shelter, hot food and healthcare," said a government source."It will take some time for the message to sink in, they have to hear it from people they trust," the official added. Over 400 people have left since Wednesday to return to the port of Piraeus - a distance of around 600km - from where they will be shared out between camps with spare capacity."I'm going to Trikala [in central Greece]," said Jalla, a 19-year-old Afghan."They told us the camps are clean, with doctors and real dormitories," she said.Giwan Al Haije and his wife Urmen, a young Syrian Kurdish couple, have decided to apply for relocation within the EU.Out of 160 000 asylum seekers set to be transferred from Greece and Italy under the relocation programme, fewer than a thousand have left so far."We want to apply for the programme because my wife fell ill at Idomeni. It was hell there," Giwan said."We've had enough of camps, having lived in one at Erbil [in northern Iraq] and another one in Turkey," he said.