I see 3 possibilities:
1) You will not be spending as much time w/ the other couple as you would like due to the 12 yr old. Because if she is a "helicopter" mom there will be no convincing her to let go of the "baby". As long as they are paying for their portion of the cruise it's up to them to decide whom they are paying for to join them on the cruise. If you paid their fare, then put your foot down that you did not pay for them so they could afford to take their children along.
2) She may already have in the back of her mind, that her 2 college aged kids will do the job of watching the younger one freeing her and her husband up to things as an adult w/ you and yours. Well as a mom of some one that just entered their sophomore year of college all I can say is I hope if this is the case that your friend already negotiated a "contract" w/ the older 2 that babysitting their brother is their "bill" for a parent sponsored cruise.
3) You or some one else will convince the mother that the Kids Camps on board are no different than a day camp or after school activity situation where there are plenty of other kids in the same age range that will be part of organized and supervised activities. If she allows the child to go to school or even participate in organized sports then she's left him in the care of others before. I think you need to show her (hint search around you tube) that she's is not leaving him "alone" or in even in solitary babysitting w/ an untrained or foreign language only speaking person. Try to find out what exactly her fear is.
Just make it clear to them that you will only be joining them when all their kids are otherwise occupied elsewhere. Perhaps let the husbands talk this over. Your DH maybe able to convince her DH that a cruise is more enjoyable to a couple if the kids are more "heard from than seen", and he should have more sway over her than you will.
Just remember even if your bookings are linked you do not necessarily need to eat dinner w/ them. You can break the the linkage, opt to contact to your PVP, TA, or even the maitre 'd about a different table, change your dinner times to something other than what they have reserved, or if you opted for My Time Dinning just go at a different time then them.
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