Under European regulations known as Dublin III, child asylum seekers can have their claims transferred to another European Union country if they have relatives there.
Longfield's office said charities claimed some of the children were as young as 10, and had "fled war and fighting in countries such as Syria".
Over one million people fleeing war and poverty flowed into Europe last year, setting the stage for Europe's worst migrant crisis since World War II.
Longfield said: "I have asked the French authorities to urgently determine which of the children in the Calais refugee camp are eligible to come to the UK, to ensure their safety and to process their applications immediately.
"It is unacceptable that at the moment some are waiting up to nine months before they learn whether or not they can come.
"The camp is an incredibly dangerous place for an unaccompanied child."
She asked the authorities in France to establish a dedicated centre to process the children's asylum applications and for them to be addressed urgently.
"Whilst in the camp, the children have been exposed to trafficking, violence, abuse and illness," her office said, adding that some had attempted to enter Britain illegally by hiding in trucks.
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Longfield's office said charities claimed some of the children were as young as 10, and had "fled war and fighting in countries such as Syria".
Over one million people fleeing war and poverty flowed into Europe last year, setting the stage for Europe's worst migrant crisis since World War II.
Longfield said: "I have asked the French authorities to urgently determine which of the children in the Calais refugee camp are eligible to come to the UK, to ensure their safety and to process their applications immediately.
"It is unacceptable that at the moment some are waiting up to nine months before they learn whether or not they can come.
"The camp is an incredibly dangerous place for an unaccompanied child."
She asked the authorities in France to establish a dedicated centre to process the children's asylum applications and for them to be addressed urgently.
"Whilst in the camp, the children have been exposed to trafficking, violence, abuse and illness," her office said, adding that some had attempted to enter Britain illegally by hiding in trucks.
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