The children are visiting Poland and Germany with their parents the Duke and Duchess of Cambridge.
The Duke and Duchess of Cambridge and their two children have arrived in Warsaw for the start of their visit to Poland and Germany.
Their five-day tour of the two European countries is at the request of the Foreign and Commonwealth Office.
Kensington Palace said Prince George, three, and Princess Charlotte, two, will be seen “on at least a couple of occasions over the course of the week”.
They joined their parents in Canada last year for an official trip.
“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 spokesman said.
The family of four was greeted at Warsaw Chopin Airport by the UK’s ambassador to Poland, Jonathan Knott, and his wife alongside Poland’s ambassador to Britain, Arkady Rzegocki.
Prince George and Princess Charlotte were last seen in public on the balcony of Buckingham Palace for a flypast following the Trooping the Colour.
By Peter Hunt, BBC royal correspondent
For Prince George and Princess Charlotte such trips are a novelty but, as the future of the British monarchy, they’ll one day become a way of life.
For their parents, the visit to Poland and then to Germany will inevitably be viewed in the context of Brexit.
It won’t have any impact on the negotiations.
It will, the Foreign Office hopes, remind people of the strength of the ties that will endure after the UK has left the EU.
It’s this mission the royals have pursued in recent months in various European cities.
Read more from Peter
The royal couple and their children were then welcomed in Warsaw at a meeting with President Andrzej Duda.
They visited the Warsaw Rising Museum, dedicated to the 1944 Polish uprising to liberate Warsaw from German occupation during World War Two.
Prince William and Catherine will also meet survivors of the Holocaust and Nazi oppression.
In Poland, they will tour the former Stutthof concentration camp where 65,000 people died, and tour the site of Gdansk’s shipyards, the birthplace of Poland’s Solidarity movement that helped topple communist rule.
In Germany, Chancellor Angela Merkel will hold a private meeting with the royal couple 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 Germany city of Heidelberg, which is twinned with Cambridge.
William and Catherine will cox opposing rowing teams in the race with crews from Cambridge and Heidelberg.
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