G.B.
I would talk to everyone who has ever been on a cruise so you can find out how vast the ship is and how each person can go and do their own thing and maybe not even see anyone they know for the whole day.
I think that perhaps if you look at it like that then perhaps you "could" see things a bit differently.
I would not have enjoyed going on vacation with my mom anywhere. But I did go several places with her on long weekends. I was miserable and swore to never do it again. My daughter remembers those times in a completely different light and it was the best times for her. That is the only reason I would suggest sitting down with the in-laws with lots of information about what happens on a Disney cruise and how much time you would be willing to see them during the day.
I would sit down with the in-laws with a long list of the activities on the ship and the islands that that particular ship would be going to then say if we plan this I would like for it to go something like this. If they are agreeable then it's more likely to happen and go smoother.
For instance. Disney has live stage shows that are a couple of hours long. You could do one of those on one day with the whole family together. Then you could meet up for dinner that evening at say 6pm. The rest of the day is separated. Kids in kids camp, in-laws doing their own thing, you and hubby having an alone day together.
The next day you may be at some island. Plan on meeting up for lunch then go separate ways again. Meet up on the ship later that night for a late drink or something.
Things to know ahead. They will want to do something with their grand-kids without you, that's a good thing for the kids. They will enjoy a treat with them.
You will get some time alone too. That will be good for you and hubby too.
Think about one of the 3 day cruises. Surely you can get lost on a huge city on the sea during the day and find some enjoyment somewhere.