We've taken our son 3 times between ages 8 thru 10 and he enjoyed the cruises greatly and wants to return.
There is plenty to do on the ships and off. He loved the White Pass RR in Skagway for one example. The kids programs on the ships were great too. He didn't always utilize them but chose the events he wanted to participate in.
If you choose a ship with a covered pool then they can swim all they want, even in cool or rainly weather.
It'll depend on the cabin size you choose as to how comfortable you will be. For the 3 of us we had a large bed for the adults and a fold-out sofa for my son. Yes it was tight but we dealt with it fine. Some ships have family cabins I believe which will hold 4 but we never checked into that.
taking the kids will be a good choice and a wonderful shared experience and vacation. Go for it.
I have taken my kids. Yours are old enough. Involve them in the planning. Budget fully for excursions- and at least do one "biggie". With a glacier helicopter landing my vote. But if you are looking at a "standard" cabin- it will be very crowded- think half of a Motel 6 room. But who cares??? I've done it may times and the money saved has bought me some great flights.