Rutherford brought son Hermes, 8, and daughter Helena, 6, to the Ovarian Cancer Research Fund's Super Saturday fundraising event.
She was "clearly delighted to be with her kids," a rep for AquaBall tells PEOPLE, adding that they seemed to have a "great time."
The family was also seen sipping on AquaBall children's water.
"They were partaking in all the different activities, visiting different booths and enjoying the day," the rep says. "At one point she took them over to a children's play area where her daughter painted and she played an interactive game with her son. She donned a flower crown at one point that she got from one of the booths."
The Gossip Girl star has her kids with her for the summer, though she has been battling to bring them home to America permanently. Since 2012, they've lived with their father, German businessman Daniel Giersch, in Monaco, at a judge's ruling.
Courtesy of AquaBall
"I can only see this as a legal kidnapping by a California judge forcing my children to live abroad for so long that the other countries seize control over their lives," she testified before Congress earlier this month.
Giersch's lawyer, Fahi Takesh Hallin, told PEOPLE in a statement that "Daniel will continue to promote Kelly's relationship with the children."
The Stars of Gossip Girl
"He believes that the children deserve to love both parents and has never nor does he intend to ever participate in any negative press directed at Kelly," Hallin said.
Rutherford's custody battle has stretched across two continents and it will return to New York after a California judge ruled that the state did not have jurisdiction.
"We are pleased that at long last this legal atrocity has been extricated from the grip of California's horrific family court system," Rutherford's attorney Wendy Murphy told PEOPLE in a statement.
"Now we hope and expect the New York courts, and Secretary of State John Kerry's office will protect the children from further harm and ensure that they are able to reside in their own country, where they belong."
Reporting by KATE HOGAN