08/31/2012 at 01:15 PM 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."
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."