I have posted before to base the amount of insurance on the highest per person cost instead of as an average of the total cost. The first 2 people in the cabin pay more than the 3rd and 4th passengers, so that's the cruise amount you should use. To the cruise fare add your airfare and any other prepaid, non-refundable expenses. Then round-up to the next $500 increment.
Example: passengers 1 and 2 pay $1000/pp for the cruise and passengers 3 and 4 pay $500/pp. If airfare is $350/pp - you want a quote based on the $1000/pp cruise cost plus the $350/pp cost of airfare, then round up to the next $500 increment. $1000 + $350 = $1350...round up to $1500/pp in coverage.
If you take the total cost of the cruise ($3000) and add the airfare ($1400) for a total trip cost of $4400 - each person gets $1100 of coverage. That leaves the first two people in the cabin underinsured by $250/pp.
*To figure out how much of the fare is for the adults and how much is for the kids - try booking a cabin for the same cruise on DCL's web site. Price it for just the adults, then add the kids and see what the difference is.