My kids were essentially the same ages as yours on our last trip, and the Joovy Caboose worked well. I did bring a soft sided carrier, and actually ended up carrying the 4 year old in it once, during a particularly intense meltdown of hers. I typically only "wore" the 2 year old in line.
For our next trip, I am considering renting a Baby Jogger City Mini Double, because our Joovy Caboose frame has gotten a little bent out of shape, which makes it more difficult to push. I also think it would be more comfortable for a larger child to climb in and out of, since we typically have a bag hanging on the back. My two older kids will be 9 and 12 and my 6 year old is has a very low tolerance for discomfort. I don't expect my 12 year old to ride at all (unless his baby brother wants to push him) but my 9 year old is prone to meltdowns.