We've taken 8 different cruises. As mentioned, Viking doesn't allow children. Princess allows children, but we saw few on the cruises we took, which we all during the school year. The Norwegian cruise had cartoon characters in costume and had quit a few kids.
Even though Norwegian had kids, it was the only ship I've been on that had an "adults only" sun deck. I was up there with another guy, when a woman around 50 showed up and warned us that the sun deck was a clothing optional deck, and whether we stayed or not, she was about to take advantage of the sun. We both quickly disappeared.