We did Rhine last summer with a 19 year old and a 16 year old - both girls. Trip selected by the 19 year old (when she was like 13). We all loved it, as well as our pre days in Basel and our post-days in Amsterdam.
We liked the mix - cathedrals, castles, cute European towns, museums (both world class and unique), the outside activities, the food (both on and off the boat), wine. There were more active days/activities available too, a few of the days.
I think that you need to just really look at the itinerary and make sure that the kids do too. See if there are things that they like to do/would want to do. The cruise didn't have as much "kids by themselves with guide" as we'd seen on our other
ABD. That was okay for my girls, as they generally wanted to do the things on the trip.
Temps - it will likely be hot and humid. It was in June last year.
My youngest liked the Rhine so much that she has now changed her first choice for her grad trip from the Poland/Prague/Vienna trip to the Danube cruise.
Good luck deciding.