Harper, who recently turned eight on July 10, seemed happy to be playing with her older brothers.
The little girl wore a light blue one-piece bathing suit and high-waisted denim shorts.
Romeo, 16, attempted to stay cool and only wore black shorts and white socks while he kicked around the yellow soccer ball.
The second youngest Beckham, Cruz, 14, wore black swim trunks and a white tank top.
David, 44, who ended his 21-year professional soccer career in 2013, also treated his kids to a day of fun at a Miami waterpark called Tidal Cove. "So much fun with the kids at Tidal Cove... MIAMI BABY," the father-of-four wrote alongside a few photos from the park.
Romeo also shared a sweet selfie of him and his father on a speed boat. Everyone but the oldest Beckham child, Brooklyn, appeared to make the family trip.
What do you think of the Beckham's family trip to Miami? Let us know in the comments below!