Ranbir says he had fun at the event at the Tata Memorial Hospital.
"This is my first time here. It is a great initiative. There is nothing better than making children smile. Smile is the most important thing," Ranbir said outside the hospital.
The Tata Memorial Hospital organised Hope 2012, Christmas celebrations for children battling cancer.
"Don't worry be happy is my message to these children," said Ranbir, who played a deaf and mute boy in Basu's "Barfi!".
The actor also danced with the children on the songs of "Barfi!".
The "Student of The Year" actor Varun also grooved with the kids on songs from his movie. He felt content.
"It felt very good. Dancing with these kids is amazing. It feels nice to be able to bring a smile on someone's face," Varun said, adding that he has resolved "to stay positive like these children", in the new year.