The royal children were smartly dressed, with the future king in a shirt and shorts and his sister wearing a summer dress.
Two-year-old Charlotte and three-year-old George have not been seen in public since their appearance on the balcony at Buckingham Palace to watch the Trooping the Colour ceremony last month.
Gosia was also in attendance at the party, and shared a photo on Facebook, writing: "In a second the Duke and Duchess of Cambridge will join us for a garden party".
Britain's Prince William took his young family to Warsaw on Monday for a four-day tour of Poland and Germany that will take in World War Two memorials and meetings with locals.
And in 2014 the young prince flew with them to New Zealand and Australia for their extensive visit. She will spend two days in Poland, followed by three days in Germany.
Fans who got a chance to speak to the royal family included 21-year-old student, Magda Mordaka, who excitedly revealed: "We are fans of the British monarchy".
The state visit is part of the royal couple's unofficial duties as Brexit ambassadors, as they attempt to strengthen the UK's relations with Europe as it prepares to leave the European Union, and comes a day after the pair were spotted congratulating tennis champion Roger Federer's win in the Men's Wimbledon Final.
Meanwhile, Charlotte held on tightly while her mother gave her a lift.
When asked if the Royal couple would come across anyone talking about the topics of migration and citizens rights, Mr Knott said: "I wouldn't say they were off the table but I don't think that's the kind of visit that we're on".