The 52-year-old "Black Mass" stars' daughter, now 16, was hospitalised in 2007 after contracting an E Coli infection that caused kidney failure. She was 7 years old at the time, reported People magazine.
"For me it's a gift. They give me the gift. When my daughter was ill in Great Ormond Street it was the darkest period of my life. I'd always done these visits but after that experience the visits became more and more important.
"The kids are so courageous but to be able to bring a smile or a giggle to the parents means everything in the world to me," Depp said.