It's often difficult for children to grasp the fact that other kids aren't as healthy and capable as they are. Your local children's hospital probably has programs designed to cater to younger volunteers, although smaller children might not be allowed to interact directly with patients. Still, many hospitals offer service projects that children and their parents can participate in, or you can shop for and donate toys or other supplies from the institution's wish list.
Many children's hospitals have programs geared toward teenagers, though. Often, these kids are interviewed and background-checked as part of the process. Once they're selected, they typically help deliver goodies or simply play with patients that need a break from the reality of hospitalization.