A baby girl and her two siblings who went missing after social workers raised concerns about their welfare were taken out of the country by their grandmother despite ports being put on alert.
On Tuesday Mr Justice Keehan made an appeal for help to find baby Rose Paduraru, seven-year-old Oliwia Ilksa and 12-year-old Krzysztof Ilksa.
The judge, who is analysing the case at hearings in the Family Division of the High Court, said he was "very seriously concerned" particularly about eight-month-old Rose.
He said they had been living with their Polish mother Joanna Ilksa in Hillingdon, west London, and had recently disappeared shortly after another family court judge decided that they should go into the care of social services bosses Hillingdon Council.
Lawyers on Thursday told the judge that the children flew from Stansted to Poland on Monday with their mother's mother Barbara Wyrzykowska.
"Despite the ports alert being served, grandmother managed to remove all three children from the jurisdiction via Stansted airport," said barrister Ann Courtney, who represents Hillingdon Council.
"All three children are now in Poland at the grandmother's address."
She said Polish police had made a "welfare check" and had not raised any concerns.
The judge said he wanted the children returned to London.
Rose's mother and Romanian father, Nelu Paduraru, who are both in their 20s, had been arrested after lawyers representing Hillingdon Council suggested they might have breached a judge's orders by not revealing all they know about the children's whereabouts and could be in contempt of court.
Mr Justice Keehan has ruled that both can be released.
Lawyers said both wanted the children returned to England.