Points cruises need to have at least the first adult in each room as all points, Additional passengers can be points or cash (you cannot split one person)
I believe that the first 2 passengers in a room are considered adults for price / points usage (so in a one room scenario you would have 2 adults and 4 children, and only the first adult must be points, but in a two room scenario you would be booking as 4 adults and 2 children and would have to use points for 2 adults)
I have not called MS to find their cash costs. When I once figured points usage of Cat4 vs (2) cat 10/11, there was not a huge jump. Although I've read there are a few adjoining cat 10 or 11's and on a cash only basis getting those adjoining rooms are slightly cheaper than a cat4, and adjoining gets you more privacy (2 rooms) and an additional bathroom, but no balcony..)