Just came across a great site online http://www.postpals.co.uk/
Post Pals is basically a service that allows the public to send mail, e-cards, gifts etc to children with chronic illnesses. You are able to view the children and read about their life and illness. Each of their pages gives details about the things they like and their interests along with information about their family, siblings etc. Interestingly, it mentions that it's a nice idea to send post to siblings and parents as they need as much support as the sick children do.
Reading through some of the stories is very moving especially on the news page where you hear of family tragedy but it certainly makes you put your own problems into perspective.
I will be off to buy some nice child-friendly writing paper within the next few days and look forward to sending my first letter.