Welcome aboard, Maria! What an amazing way to spend your parents' 50th wedding anniversary! Be sure to call Disney Cruise Line ahead of time and let them know you'll be celebrating the anniversary onboard. They can note this in your reservation so your servers and other Cast Members will be aware of the big anniversary!
While I haven't taken the Alaskan cruise myself, I have heard amazing things about this voyage. Take a look at
Disney Mom Andi's videos about the Disney cruise to Alaska (click on the Discover Alaska tab). This will give you some insight into what to expect.
Your older children will not be bored! We sailed with my brother when he was 26 - a young single guy. He had so much fun at the activities and night clubs. There is something for every age on the ship. I even remember a few nights when there were activities just for the young 20's crowd. Schedules are always subject to change, so I can't tell you exactly what they can expect, except that they will certainly enjoy the night clubs onboard. They are open until the wee hours, have great events for the 20's crowds and your children will be thrilled to participate.
Have fun viewing the Alaskan cruise videos as you continue to prepare for your own Alaskan adventure this summer! Let me know if I can help with anything else before you set sail, Maria!