The 34-year-old took to Twitter to share his experience, reports contactmusic.com.
"Such an incredible time at Great Ormond St Children's Hospital. Such brave kids," he tweeted.
Actor-model Colton Haynes also accompanied Morrison on the trip and uploaded a photo on Instagram of the pair posing with a female patient.
He added: "Had the pleasure of meeting Rebecca today with Matt at our Great Ormond Street children's hospital visit. Sending more love Bec."