More than 10,000 migrant children may have disappeared after arriving in Europe over the past two years, the EU's police intelligence unit says.
Europol said thousands of vulnerable minors had vanished after registering with state authorities.
It warned of children and young people being forced into sexual exploitation and slavery by criminal gangs.
Save the Children says some 26,000 child migrants arrived in Europe last year without any family.
It is the first time Europol has given a Europe-wide estimate of how many might be missing.
"It's not unreasonable to say that we're looking at 10,000-plus children," Europol's chief of staff told the Observer newspaper.
"Not all of them will be criminally exploited; some might have been passed on to family members. We just don't know where they are, what they're doing or whom they are with."
Officials in Italy warned in May 2015 that almost 5,000 children had disappeared from asylum reception centres since the previous summer.
In October, the authorities in Trelleborg in southern Sweden said about 1,000 unaccompanied refugee children and young adults who arrived in the town in the previous month had since gone missing.
Confirming the overall estimate of missing minors, a Europol spokesman said a large proportion may have also disappeared after landing in Greece. The country is the first entry point for most of the 1 million migrants who arrived in Europe by boat in 2015, and authorities have been criticised for failing to register and check the arrivals.
Criminal gangs known to be involved in human trafficking in Europe are now targeting refugees, Europol said.
There are fears unaccompanied children and young people may be dragged into sex work, slavery and other illegal activity.
The International Organization for Migration (IOM) spokesman Leonard Doyle told the BBC the figure of 10,000 missing children was "shocking but not surprising".
He said it was "to be expected" that many of these would be caught up in exploitation.
"Let's hope now the EU puts the resources into finding these children, helping them and reuniting these children with their families."
The warning from Europol comes days after the UK government said it would accept more unaccompanied child refugees from Syria and other conflict zones, without giving numbers.
However, it said it would not be taking in vulnerable children who had already made it into Europe.
On Saturday, at least 39 migrants, including several children, drowned trying to cross the Aegean Sea from Turkey to Greece.
The IOM said on Friday that 244 migrants had drowned in the Mediterranean so far this year, out of 55,568 arrivals.