Travolta, Preston and daughter Ella Bleu, 10, spent time with the children whom the foundation supported over the weekend.
The fund was set up by Mandela — the first black South African president —to provide services for less fortunate children.
Travolta, who spoke about how children "keep you humble" and "grounded," urged other celebrities to use their status to contribute to Mandela's legacy.
The 56-year-old actor and Preston, 47, recently announced that they are expecting. The couple lost their son Jett when he died of a seizure in 2009.