Alaska is great for kids - I'm sure they'll have a wonderful time!  Over the summer, there will be a lot of families on a celebrity cruise to Alaska, so they'll be fine.  Just take them to the kids club on the first night (or sign up night) so they can meet the other kids their age, and the 13 year old might prefer to wander the ship with their new friends or go to the teen club to meet the others.   Alaska shore excursions can get pricy, so just try to book directly with the providers