1. When to arrive, where to Stay
Always come in the day before your cruise - especially if flying. A hiccup in San Francisco might impact the flights from wherever you are coming from. And the airline don't have any excess capacity in case of issues - "If the Plane Isn't Sold Out - We're NOT Flying IT!!" (I don't know what / how they will get all of those folks out of Chicago from this weekend confused3:
a) Stay wherever you like. WDW resort, airport area hotel, Cocoa Beach. Lots of choices. Last time we cruised, we got a
"package" that picked us up from MCO and took us to the Cocoa Beach area hotel, the hotel, transfer from Hotel to Ship, and at the end of the cruise, ship back to MCO. Prices seemed reasonable.
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2. Ship .... They all are great. Each has their own personality. Can't go wrong with any of them.
a) your crew:
- the Adults = any ship,
- the 18yo will be considered an adult and not be allowed into any of the clubs. there's the "1820 Society" that has scheduled informal gatherings of the kids that are 18-20yo. Any ship will be OK for them, but the bigger the ship, the better chance at actually having a group so lean toward Dream / Fantasy
- the 12 yo would probably love the "extra" stuff the Dream / Fantasy have: Mickey's Mid Ship Detective Agency, AquaDuck, Mini Golf. If Star Wars fan one of the ships has the Millennium Falcon.
- the 3yo won't care which ship - will probably love the kids clubs on any of the ships, but the Magic / Wonder are smaller and more intament and might be a better fit with the smaller number of kids
A question for YOU.
How Long of a cruise do you want: Dream is 3 or 4 nights, Fantasy is 7
In April the Magic and / or Wonder might be sailing there on certain itineraries - varying in length from 3 to 7 nights. Some cruise might leave out of Miami.
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3. Want "Less Bumpy"
then probably go lower and mid ship. Deck 2 is terrific. Inside and Large Porthole rooms.
4. Passports: No, BUT...
They are designed for travel, Birth Certificates are not. If you are doing a closed loop cruise (start and end in the same port), Government Issued Photo ID and BC is fine. I travel with my passport now - even for flying - it's just easier. If you do have passports, be sure to bring your Gov't Photo ID (drivers license) because for getting back on the ship, it is easier than your passport.
Passport are good for 10 years so if you think that you might need them in the future (ie: college - study abroad, changing job - completing the I9 form, etc) the cost per year is not that bad (just all of that up front money!!) The 3 and 12 yo passport will only be good for 5 years. At some point I expect that Passports will become the ID of choice for many activities (flying, cruising, voting, etc).
5. Tips for Boarding:
Online check in. For new to
DCL cruisers (which I think you are) is about 75 days before sailing. During the process, it will ask for your ID number, How you will pay for onboard "incidentals", and more importantly WHEN you want to arrive at the Port (Port Arrival Time (PAT)). Boarding usually starts between 11:30 and noon and goes until about 3pm. To maximize your time onboard you would want to pick the earliest PAT available. If you want to "walk in / walk onboard" you would pick a PAT between noon and 1pm (lunch is served until 3).
6. Other Thoughts
a) You Can't Eat It All, but try other foods than normal. If you don't like, they will bring you something else - for FREE
b) You Can't Do It All, but sleep is for after the cruise. The hardest part of a DCL cruise is deciding what NOT to do.
c) Bring and USE Sunscreen - the sun

is different there than at home,
and you can get burnt in a hurry!!
There are "Other Costs" associated with the cruise, specifically, Gratuities. These are given to 4 people that take great care of you throughout the cruise: The Head Server (manager), the Server, Assistant Server and your room steward. These gratuities are based upon the length of the cruise and the number of guests in your party (5 in your case). You do NOT tip at each meal. There is a "standard" amount based on your cruise. Budget for this before you leave home.
Shore Excursions: with DCL you can reserve these during your online check in. Unlike other cruise line you will NOT pay for them up front when reserved, but rather they will be charged to your onboard account. Remember this when budgeting.
Souvenirs onboard and on Castaway Cay will be charged to your room.
Adult Beverages, and other "made to order" beverages (smoothies) all will be charged to your room, as well as soft drinks ordered in a lounge / bar / entertainment venue or in room service.