It's all about personal preferences.
We always cruise with the four of us (me, my husband and two sons, ages 11 & 13 yrs.) Our "ideal" set up is one balcony cabin for the boys and an adjoining balcony cabin for us. We could never squeeze into anything less. If we can't get the 2 adjoining balcony cabins, then our second choice is a suite.
This is what we feel we 'need' to be comfortable, but not everyone has the same requirements or tastes. I've known a family of 4 that had a reg. balcony cabin and have survived, but I wouldn't want to try it myself.
If you can, get an inside cabin (at least) for your son and mother-in-law, perhaps across the hallway. I think you'd be happy to have the extra bathroom, extra TV and extra storage space. Believe me, it wouldn't be a "waste of money".
You don't realize how tiny these cabins are until you see them. I'll never forget my heart dropping when we went on our second cruise. Our first was in a suite. Our next cruise was in a mini-suite on the Grand. I just about died when I saw how narrow the cabin was. I wanted to run the other way and tell them they made a mistake. "It looked so much bigger in the brochure!", I said. Obviously, our next cruise was in a suite. :grin
