The cheapest (IMO) 2 rooms: 2 adults & one of the children booked in one; 1 adult and other 2 children booked in the other. You must make the reservations with at least one adult in every room.
Couple of things: Once onboard, you can switch rooms to sleep however you wish. And, in the second room, one of the children will be charged full adult price (first 2 people in each room = full adult price). The 3rd person is cheaper (based on age).
Or you can go for 3 rooms, but everyone will pay full adult price then.
I'd suggest calling DCL direct so they can help you juggle the numbers/costs.
If one of your children wants a top bunk, I would suggest making sure 4 fit in the rooms you selected or book two adults and 2 kids in one room and one adult and one kid in the other to guarantee you get the pulldown bunk bed
Here's what we did - 2 Adults, 4 kids, one of whom is 22 for our upcoming Magic cruise. We booked two in a Veranda (5C I think), and four in an Interior (11B I think) right across the hall. When we get to port we'll sort out the access cards so everyone has access to both rooms. We figured that the interior would be the sleeping room, and the veranda would be the hangout room most of the time anyway. That saved us some money and packed as many people as possible into the cheaper room. Once we get aboard - Disney doesn't really care who sleeps where. I know the 5C room sleeps 4. If muster stations haven't changed in 7 years on the Magic, then I also know that we'll all be in the same muster station
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