Haven't used the CARES harness, and haven't been to WDW yet, but we did travel a fair bit with our toddler last year (England, Boston, Cuba)... my vote would personally be a no on the Trunki or any other kid luggage. My experience lugging 2 preschoolers through multiple airports is that they are all excited about the luggage for about 5 minutes and then it becomes mom or dad's problem. I would much rather have a "normal" sized carryon/backpack that is easier to handle for *me* and use an umbrella stroller/kid leash/carrier depending on your kid's size and tendency to bolt. My DD is just about 6 and I still wouldn't give her anything much larger than a toddler sized backpack until recently. Now, she will likely take her school backpack as her carryon and be responsible for it.
On this trip, since we are planning on renting a car midway through, we are taking our carseat for DS. If that wasn't the case, I'm not sure I'd bother with the CARES harness. But you know your own kids best - mine are pretty good at sitting as long as they have something to occupy them.