The third and fourth in line to the British throne joined their parents -- the Duke and Duchess of Cambridge -- on the five-day tour of Europe undertaken by the royals on the request of the UK's Foreign and Commonwealth Office (FCO).
The visit coincides with the second round of talks between officials in Brusselsover the contours of Britain's exit from the European Union (EU).
"The Duke and Duchess are very much looking forward to this tour and are delighted with the exciting and varied programme that has been put together for it," a Kensington Palace spokesperson said.
This the second time the young royals will be on an official charm offensive, after their tour of Canada last year.
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Britain's royal couple and their children will be welcomed first in Warsaw by President Andrzej Duda.
They will visit the Warsaw Rising Museum, dedicated to the 1944 Polish uprising to liberate Warsaw from German occupation during World War Two.
Prince William and Kate will also meet survivors of the Holocaust and Nazi oppression.
In Germany, Chancellor Angela Merkel will hold a private meeting with Prince William and Kate in Berlin before they visit the Brandenburg Gate, a symbol of German unification.
The Duke and Duchess of Cambridge will also visit Berlin's Holocaust museum and memorial.
A boat race is planned in the city of Heidelberg, which is twinned with Cambridge.
William and Kate will head opposing rowing teams in the race with crews from Cambridge and Heidelberg.