Teresa Quiej-Alvarez, playing with Mel Gibson
Jae C. Hong/AP
08/07/2011 AT 01:30 PM EDT
Mel Gibson and his estranged wife
Robyn made a rare public appearance with three of their seven children and three of their grandkids on Saturday.
The special occasion, taking place in the hills of Malibu, was the 10th birthdays of two young Guatemalan girls close to the family's hearts: Maria de Jesus and Maria Teresa Quiej-Alvarez, the formerly conjoined twins
connected at the skull who made international headlines when surgeons at Mattel Children’s Hospital at UCLA Medical Center separated them in August 2002.
"We're all involved in Josie and Teresa’s lives," Gibson, referring to the girls by their nicknames, tells PEOPLE. "It's incredible seeing Josie and Teresa do so well. Listening to Josie speak, you think, 'Man, this is the happiest kid I’ve ever seen.' "
The Gibsons appeared in good spirits, as well. Amid bongo and steel drums, hula dancers and 100 guests at the Hawaiian-themed party, Mel and Robyn, who arrived separately, mingled individually with friends, doctors and executives for the hosting charity, Mending Kids International
Midway through the event, Mel, 55, went over to say hello to Robyn and to wiggle the toes of their 5-month-old grandson.
"He likes that," Mel said with a chuckle. During the birthday cake cutting ceremony, Robyn and Mel were honored for their generous support of Mending Kids, which provides life-changing surgical care for children worldwide.
With Robyn formerly having been president of the board of directors of the nonprofit group, Mel said: "My wife was really far more involved than I was in the past, but we're both still involved."
Recently returned from two Mending Kids medical trips to Guatemala and Panama, Mel also said, "We gave them financial support, but it was hands-on sometimes. I remember Robyn taking care of children who came here [for surgery]."
He added: "All of my kids do give back. Our children have been raised that way and that will be an integral part of their lives as they move on."
Gibson, whose next screen project, the dark comedy How I Spent My Summer Vacation
, is due out next year, said of working with the organization: "It gives you perspective … It's one of my faults, you tend to focus on yourself a lot. Which is not always the healthiest thing for your psyche or anything else. If you take a little time out to think about other people, it's good. It's uplifting."