Brad Pitt enjoyed an artistic outing with his two youngest children in London on Saturday.
Pitt, 52, who's currently in the British capital filming an as-yet-untitled World War II spy drama costarring Marion Cotillard, took Knox and Vivienne, 7, for a special pottery-painting treat in the cold winter sunshine – and even picked up a brush to make a creation or two of his own.
"It was just Brad and the twins doing the painting together," a source tells PEOPLE of the excursion, which happened a day before the U.K. celebrated Mother's Day. "They were in the pottery cafe for a couple of hours and seemed to have a lovely time. They were very happy."
The source added: "It wasn't a Mother's Day thing. Knox and Vivienne painted a few things and then Brad painted a few things himself. It was just a fun day out with Dad."
Earlier in the day, Brad was also spotted walking around west London with the twins and their eldest brother, Maddox. The Cambodian-born 14-year-old has recently been helping mother Angelina Jolie Pitt with some movie research for the real-life Cambodian drama First They Killed My Father, which Jolie Pitt co-wrote, produced and directed. His Vietnamese-born brother Pax, 12, also took on the role of set photographer.
Based on the harrowing memoir of Cambodian author Loung Ung, the drama, which will premiere on Netflix later this year, details the trauma of war from the perspective of a young child – a cause that has grown increasingly close to Jolie Pitt's heart as a result of her extensive work as a U.N. Ambassador.
"I was deeply affected by Loung's book," Jolie Pitt said in a statement. "It deepened forever my understanding of how children experience war and are affected by the emotional memory of it. And it helped me draw closer still to the people of Cambodia, my son's homeland."
Jolie Pitt is currently putting the finishing touches on the movie in post-production.
Yet it's not all work and no play: The mom of six has also been busy enjoying time with Pax, Zahara, 10, and Shiloh, 9, in Los Angeles.