Jessica Simpson rocked the house on Sunday afternoon, but the audience wasn't her usual crowd.

Instead, the singer took the stage to perform for about 2,000 pediatric cancer survivors for the 18th annual Celebrate Life with Hope event.

"To be able to give back is my sole purpose in life," Simpson says about why she decided to get involved with the event, which benefits the Childrens Center for Cancer and Blood Diseases at Childrens Hospital Los Angeles.

Together with event planner Mindy Weiss – who planned both Simpson's wedding to Nick Lachey and Ashlee Simpson and Pete Wentz's wedding – the singer made sure guests had a great time.

"She was so nice and so warm. All the kids were getting a day out and the little ones were looking up at her, it was just really touching," says a guest. "She kept telling them they were her heroes."

Simpson performed four songs, and brought her dad along as well. "At the end, she had a little meet and greet up on the stage," the guest added. "The kids were just so excited to see her and give her a hug and she was so sweet with them. She was signing T-shirts and the hospital gave her a plaque. She was just really happy to be there."

The Celebrate Life with Hope event is part of National Cancer Survivors' Day across the country, and is the largest such event for pediatric cancer survivors in the Western United States.