Zemborain told the Associated Press that Francis told her that they were crazy to drive so far with their children. She said it was like meeting an old friend and Francis hugged the children.
"We couldn't believe it. They called us this morning (and) we were like in shock," she said after Sunday night's Mass. "It is like being with an old friend. He was so warm. He told us we were crazy. He made jokes.
"The children got to hug him a lot. They couldn't leave him. Not at all protocol, not at all formal, it was like being with a friend."
Walker and Zemborain quit their jobs in food service and marketing to lead their children on the unforgettable tour of the Americas, using savings and soliciting donations to fund the trip to the World Meeting of Families in Philadelphia.
Along the way they made 12 border crossings and stayed with dozens of host families, did lots of sightseeing and documented the trip online. They schooled their children — Cala, 12; Dimas, 8; Mia, 5; and Carmin, 3 — with the help of a distance learning program.
The family said they got a call at 6 a.m. Sunday that Francis wanted to meet with them at the Saint Charles Borromeo Seminary, where he stayed this weekend.
Francis told the family he had been following their trip, they wrote in a Facebook post.
Zemborain, Walker and the kids plan to continue traveling until November, when they will fly home from Miami. They were going to send the van by ship, but then a relative volunteered to drive it back to Buenos Aires from Florida.
Francis celebrated Mass on Sunday that drew hundreds of thousands to downtown Philadelphia.
The Associated Press contributed to this report.