Hello nhmama, and welcome to Cruise-Chat.
I've been on Sapphire Princess several times, most recently in October, and by coincidence, in cabin E 731. It is a very convenient area of the ship and I'm sure your family will be very comfortable in the mini-suite.
We have not cruised with (young) children; but we became very good friends with a family with five children, ages 6 to 17 and they raved about all the activities for all their children, and how the staff went out of their way to make the children feel at home--especially the dining room staff.
I would not call Sapphire Princess "glass crazy"; but there are certainly lots of comfortable areas inside and out from which to enjoy the views. I believe you will find your own large balcony to be one of your favorite places to watch the scenery glide by.
As for Traditional Dining vs. Anytime Dining; my wife and I have always preferred Traditional Second Seating. It seems more relaxed, quiet, and we like having the same wait staff and tablemates for the duration of the cruise. But I think when we travel with our grandchildren we will probably opt for Anytime Dining. 8:15 seems late for children to be having dinner; and our friends with the large family said they really appreciated the flexibility of Anytime Dining.
I have never cruised with Royal Caribbean so I can't comment; but I have been a satisfied Princess cruiser for over 30 years. And Princess has an excellent reputation for its CruiseTours.
Our friends with the 5 children asked the kids what they wanted to do next year: go to Walt Disney World or take another cruise; it was unanimous--they all wanted to take another cruise.
Have a wonderful time.