Jon Voight (far right) with grandkids (from left) Zahara and Pax
08/30/2011 at 03:00 PM EDT
While mom Angelina Jolie
and dad Brad Pitt
were otherwise engaged, their children took time Saturday to hang out with their grandfather, Jon Voight.
The Oscar winner, 72, took Shiloh, 5, Zahara, 6 and Pax, 7, out to stretch their legs on Richmond Green, a grassy area not far from their rental home in the London suburb.
"There was a cricket match on so they just stopped to have a look," an onlooker tells PEOPLE. "Jon explained to the children what was going on [with the game] … it was a really traditional English scene."
Next up: a frozen summer treat.
"Then they just crossed the green and went to the ice cream van on the corner," the source added. "They seemed really close and happy, and Jon's [companion] was throwing Zahara's teddy bear in the air and pretending to make it fly. It was all just lots of kiddie stuff and the children really seemed to be enjoying themselves."
The family – in the U.K. while Pitt films World War Z
– have been splitting their time between Richmond and their temporary home in Scotland
"Angelina is now doing bits of work herself, and Brad is obviously busy with his filming, [and] there are obviously times when the childcare clashes," the source says. "So it's a case of Grandpa to the rescue!"