For a period of time we had to sail during the Northern Summer School holidays on Seabourn so have sailed on some B2B short cruises with many children. Having said that though some of the worst behaviour I have seen on those 7 day cruises were groups of adult friends travelling together. It is why we now, since we can, avoid 7 day cruises at that time.
I'm a Marriot member so I will try them at some point out of curiosity.