PIC: Jennifer Lopez's Man Casper Smart Plays With Her Daughter Emme, 3

Stepdad-in-training?

Jennifer Lopez and backup dancer Casper Smart have been dating for merely a month, but she didn't hesitate to bring her sexy beau along for a Thanksgiving vacation to Kauai, Hawaii -- where he quickly bonded with Max and Emme, her 3-year-old twins with ex Marc Anthony.

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In pics exclusive to Us Weekly, a topless Smart, 24, playfully holds Emme upside down as they frolic on the beach as mom Lopez, 42, lingers behind in the sand.

"Casper was great with the kids," an eyewitness tells Us of the dancer, who spent six nights with Lopez, the twins and other family members, including the singer's mom, Lupe.

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During the Thanksgiving Day beach romp, "Casper twirled Emme and Max around in the water" and "the twins played tag and chased Casper!"

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For more on Smart's bonding time with Lopez's cute kids -- and what their dad Anthony thinks -- pick up the new Us Weekly, on stands Friday.

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