I agree that infant travel can be very easy. (I didn't realize how easy until my son hit 3 yo!) After 3 mo, I think most parents have their child's personality figured out and all the pieces (eating, sleeping. etc) have fallen into place.

A big factor in how well the trip will go is parental behavior and attitudes. If both parents are up for the task and on the same page, things will go FAR smoother. If, on the other hand, one parent is freaking out because it is 5min past nap time, that gets the friction ball rolling and kids pick up on that.