As a child it was very, very challenging for my parents to take us places in the extreme heat since I was so weak and frail. I could not keep up with my siblings and from being unable to eat it became a challenge for my parents to try to plan anything worthy other than a trip to the beach and/or a pool. For us, almost anything we did as a family was a treat but what I remember most is feeling sick most of each trip and wondering how long it would be before we return home. As a parent ,I really never had to experience travel with an ill child since our son was and still is the picture of health.
Some advice for parents traveling with a very ill child. Please be consider to that child as well as the siblings who have to deal with it all. Do not plan unrealistic trips like never ending days dragging them all over when they would much rather be splashing in a pool. Bring plenty of snacks and drinks in a cooler even though they will want other things, these come in handy when you least expect it. Make it simple in accordance with age. (I remember dragging our son who was about 3 1/2 to Disney World and expecting him to be on his best behavior for a 10 hour day. What could I possibly be thinking?) When I loosened up and took swim breaks and just play quietly breaks we all ended up happier, and he was healthy, just too young. Think it through, google other things to do and remember they are easily entertained. We do not have to show them the world too early. They have their entire lives. 
