When you make the reservation, put one adult and one kid in one room, and the second adult and three kids in the other room, assuming you do adjoining rooms.
DCL doesn't care where you actually sleep. I say split the kids 1/3 just to make sure that one of the rooms has the top bunk which will sleep 4 total. I don't know if all the adjoining rooms do (maybe someone here knows.) You'll get better sleep, or adult time, because all the kids can sleep in one room, you can close or just prop, or even leave the adjoining door open. There is also a curtain between the bed and the sleeper bed/bunk area if the kids want a bit of privacy or light blockage. Beyond adjoining rooms or crazy expensive suites, your only other choice is two non-adjoining rooms. Then either the adults don't get to sleep together, or the kids are on their own after you tuck them in - neither are ideal with your age range of kids and if you want to actually see your spouse on vacation.
We have 4 kids too (11, 14, 17, 17), and travel with grandparents. I found it easier to just call DCL, with the website open in front of me so I could look at room locations, and pick rooms that way.
The DCL rooms are spacious enough that with 4 little kids and two adjoining rooms, it would practically feel like a palace. LOL
We did a Merry Time cruise last year. I quite liked not having to cook and clean over the holidays. My kids were a bit discombobulated by being away from home though. We surprised them and I think perhaps they needed time to mentally adjust. As mentioned, MT cruises are significantly more expensive. If that is what you want to do, and are OK with it, then my advice would be to not look at prices again or compare them to non-MT prices. Like any other discretionary spending, decide what you can spend and don't look back.