Jennifer Lopez, Casper Smart Have a Pool Party with the Twins

By Linda Marx

UPDATED 08/31/2012 at 01:15 PM EDT • Originally published 08/31/2012 at 11:50 AM EDT

Ending her world tour with Enrique Iglesias in Miami this weekend, Jennifer Lopez made a splash Thursday in the SLS pool on South Beach with Casper Smart and her adorable twins Emme and Max.

Clad in a sexy strapless yellow bikini with her hair up in a bun, Lopez, 42, spent time in the pool with her 4-year-old children and a smiling, doting Casper, 25.

"It was very hot and humid when the couple and her kids were swimming," a source tells PEOPLE. "Casper looked like he was really into playing with the kids in the pool. They looked like a happy family."

RELATED: Jennifer Lopez & Casper Smart Take Her Twins to the Park

Smart showed off his strengths, carrying Emme on his shoulder and launching her into the air. Wearing a cute red-and-white bikini, she was doing little dances and prances, smiling and having a blast, says the source.

At one point, the children were playing with swim toys while Lopez and Smart were teasing each other and laughing on a lounge.

"They were kidding each other, making jokes and enjoying the late summer afternoon," the source adds. "They looked both energized and relaxed."