It was hard to tell who was enjoying themselves most as the 25-year-old dancer joined his girlfriend and her children on a fun-filled day at the beach.
Four-year-old twins Max and Emme – from Jennifer's marriage to Marc Anthony – giggled with delight as Casper helped bury one of their security guards in the sand.
they are clearly perfectly at ease with their mum's boyfriend of seven months, who has been spotted on several other family outings.
Jennifer, clad in a bikini and with a jumper featuring the words 'kisses in the sunshine' wrapped around her waist, smiled as she snapped pictures.
The 42-year-old's mum Guadalupe joined the couple on Rio de Janiero's Ipanema Beach, in Brazil.
Jennifer was enjoying some down time on the South American leg of her world tour.
She looked to have made a recovery after injuring her back on Thursday.
It wasn't an energetic dance move that had the Jenny From the Block singer in pain, though. She strained herself carrying her children, as the star explained on Twitter.
"Got an xray this morning...all good...pulled back muscle. Coconuts getting too big to carry both at once. Awww." she posted, with the hash tag 'time flies'.