K.C.
We are taking our first Disney Cruise this fall and I can't wait. I am on a great message board called disboards.com that is all about Disney, specifically with a cruise line forum. Your kids are definitely not too old at all. Many adults travel without kids because they love the cruise line and the level of customer service, not to mention the high quality entertainment and amenities. The kids club is awesome and there are pre-teen clubs and activities too that your 12 year old would love (there are also teenage ones if he is 13 by the time you go).
8 night is a great amount of time. if you do a shorter one (5 nights or less) it often feels too rushed. you want to time to relax and enjoy both the sea days and the port days.
As for spending, it totally depends on the time of year and the type of cabin you choose. Tips are extra, and they can add up to a few hundred dollars, so factor that in. All food is included but most alcohol is not. Most activities on the ship are included but no activities off the ship are included.
have fun!!