As a teacher,

the answer depends on you and your children more than the school in this case. I have to say I think that 2 weeks at a time is 70 hours (slightly less, due to lunch, etc.) of missed instruction time, which is *very* valuable time. The only way that this can work out is if as a parent you are committed to spending time before, during and after the cruise working with your child to make up the time that was missed. If a parent simply expects the child to "catch up" that will not work. Teachers simply cannot add an extra 70 hours of their time to spend with a child to replace that.
I applaud the OP for thinking ahead of time and for your concern about your children's grades and I do not think your children should have a problem catching up. Enjoy your cruise! I've given you my suggestions on how to do this successfully.
As far as handling it with the school, this is what I would recommend. At least 3 weeks in advance, send in a letter or make a visit with the teachers and principal (or vice principal) and give them the dates that you will be vacationing. Some will *greatly* look down on this. Explain that you will old your children responsible for the missed work and will also be educating them while they are there. Life experience is valuable and Disney is certainly more educational than many other trips! Most will ask that you keep a journal each day (a few words, maybe pictures or drawings for 1st grade) about the experience. Maybe you could offer to do this if the principal is giving you a particularly hard time about it.
Go to each teacher individually at least 2-3 weeks in advance, explain the plan and *ask* them nicely what they would prefer- to give assignments early or to make them up upon return. I usually do a combination. I try to envision exactly where we will be (in 4 or 5 weeks!) and give them the bulk of the work. Often my estimates may be off. The students may have gone through the work faster or slower than I estimated. Teachers have plans, but *always* are adjusting due to faster or slower comprehension, assembly schedules, fire drills, and basically everything you could ever think of! It can't be an exact match. Quizzes, etc. will *have* to be made up for the older one.
Again, I am certainly glad you are thinking ahead. I can tell you that as a teacher I have seen (but not believed!) students absent for 1 week, 2 weeks, secretaries and attendance officers filing truant slips, worried something happened to the child, or that they are cutting, calling home, only to find that the child went on vacation and NO ONE told the school!!! (Probably thought it wouldn't get approved?) What a waste of time and money! And it could have been avoided by one letter! Of course, these students did not get any advance work, generally little to no help from their parents and have a *terrible* time catching up. I had one student who was failing 4 subjects in high school. He was suspiciously absent a few weeks ago, several days in a row. The truant process was going every day for him. Finally, we found out he had been on vacation- for 2 weeks!

He threw away his chances of passing any of those 4 subjects and will be repeating freshman year.
I am glad that most parents are more responsible and cautious than this. As long as you take school seriously your child will. Talk to the teachers. They will definitely help and I am sure you will be able to have a lovely educational trip!
Then you can go back to packing/unpacking!
Good luck with it all!!!!
