i think you can answer the question yourself. you know your kids better than anyone else so i don't know if the trip would be too much for them.
they are twins, who else do they need to play with? just kidding, i am sure there are other kids on the ship.
i think the decision is all on your side, whether or not you want to take along two 5 year olds, who might or might not (again you have to know that) are driving you crazy once they are away from home or a familiar surrounding for a longer period of time.
whatever you do, cruising the Med is definitely a nice thing! beautiful places to see and visit.
i don't have kids, but i could imagine, traveling that far with two 5 year olds could put some stress on you and your wife. (again you know your kids, I don't).
Smooth sailing Joe