According to the Australian media, Isabella, 13, and Connor, 11, arrived via private jet Thursday from Tokyo, where they had been with their father, Tom Cruise, on a publicity trip.
The kids were driven to Kidman's harborside mansion, which she once shared with Cruise, accompanied by their grandmother Janelle.
All three, along with Kidman, soon headed to Sydney Children's Hospital, where they spent about two hours chatting with patients and officials at the oncology, hematology and neurology wards, the Associated Press reports.
"I was stunned she came in today," said Sydney Children's Hospital Foundation chief executive Elizabeth Crundall. "To actually take the time out from her own wedding for a couple of hours was stunning, extraordinary and wonderful."
As Kidman and family left the hospital, photographers, camera crews and fans struggled to get a glimpse of the star, at one point blocking a hospital entrance.
"If everyone can just not get too dangerous …" Kidman pleaded.
The bride-to-be, who is expected to marry Keith Urban this weekend, has reportedly asked wedding guests to donate to charity rather than buy gifts.
TRACEY NEARMY / EPA / Landov