You've received good information already. I'd like to add some thoughts about things you may not know to expect.
What's good about your plan:
- Taking a short cruise first is a good plan because you'll learn whether you personally like it -- everyone here, of course, enjoys cruising, but I do know people who wouldn't be caught dead on a ship. We did a 4-night first to "test the waters", and we weren't sorry!
- A cruise has a lot of "moving parts" -- booking, boarding, excursions, etc. -- and you'll learn your way without involving the kids.
- Once you've cruised, you'll be a member of Crown & Anchor, and that'll qualify you for some discounts for your longer, "real cruise".
What's bad about your plan /what you can't know yet:
- Boarding the ship isn't such a great experience. About 45 days out, you'll need to sign up for a boarding time -- everyone wants the earliest time slot. The waiting in line, the going through security. It's tedious, and not everyone is on his or her best behavior. Then once you're on the ship, your room won't be ready until 1:00-2:00 (the cabin stewards have to turn over ALL the rooms in one day AND get the luggage delivered to your hallway -- I have no idea how they do it), so people are all congregating in the public areas -- with their carry ons. Those first few hours give you the false impression that the cruise will be packed every minute.
- Likewise, the last day isn't so great. As soon as the ship docks, they start pushing you off -- they're all about getting ready for the next cruise, and you'll have the return trip ahead of you.
- If you're doing a full week, you can deal with the first day being a pain and the last day being so short /rushed. But if you do 4-5 days, your proportion of fun, relaxing days will be smaller. This is one reason people do Back-to-Back cruises; it means they only need to embark /disembark once, but they get two weeks of cruising.
- Knowing this ahead of time and going in with the idea that you're "testing the waters" to see if it's right for a full-fledged family vacation, you'll understand what you're getting into.