Wearing a long flowery maxi dress, the 47-year-old actress was blooming as she joined John to give a £6,300 donation to the Nelson Mandela Children's Fund in Johannesburg.
John and Kelly met youngsters helped by the charity, founded in 1995 by then South African President Nelson Mandela to help change the way society treats children and youth.
"Children keep you humble. Children keep you grounded," the Grease star told the assembled guests.
John and Kelly's 16-year-old son Jett died last year after suffering a seizure at the family's Bahamas holiday home.
Last week Kelly's mum Linda Carlson spoke about her daughter's pregnancy to Us Weekly magazine.
"We are over the moon. I've been praying that this would happen, especially since Jett's death.
"Anything to get them out of their sorrow. I know Kelly has been longing for another baby… It's taken them out of such deep grief and made them able to focus on new life."
Accompanied by ten-year-old daughter Ella, the couple are in South Africa for a few days to cheer on the Australian football team.
Qualified pilot John, who flew the family into Johannesburg in his own private Boeing 707, is an ambassador for airline Quantas, sponsors of Australia's national side.
On Friday the 56-year-old actor met the 'Socceroos' at their training camp in Muldersdrift, near Johannesburg, where he was presented with a team jersey by players.