Welcome aboard, Dianna! My kids have loved the Youth Clubs aboard Disney Cruise Line's ships since they were old enough to attend! I remember feeling a bit worried, myself, about my three year-old playing in the same area as 12 year-olds, but was relieved when I found out how wonderful the clubs are for all those ages.
There are different activities listed in the Personal Navigator (or on your
Disney Cruise Line Navigator App) each day that take place in the
Youth Clubs, however, that is only one activity in one part of the club. The other club spaces have free play and there are plenty of Cast Members around to not only lead the scheduled event, but also others who are playing and observing in the other areas of the club.
You didn't mention which ship you'll be on, but let's use the Magic for example. There are five sections in the Oceaneer Club there: Andy's Room from Toy Story, Tinkerbell's Pixie Hollow, Avengers Academy, Mickey Mouse Club, and the big open space in the middle for large group events. Little ones who enjoy dress up play and crafts tend to gravitate toward Pixie Hollow to immerse themselves in a fairy wonderland, while older Avengers fans hang around the Marvel superhero excitement. The littlest sailors love Mickey Mouse Club where Disney Junior fun is the name of the game, and all the kids love sliding down the fun Slinky Dog slide in Andy's Room.
The biggest of the big kids in this age range tend to gather over in Oceaneer Lab more than in the club, as there are older kids' activities there like the Animator's Studio and lots of immersive virtual game play. I've never felt uncomfortable leaving my little guys when they were younger, especially when they had had such fun and were begging to go back!
Have such fun on your cruise, Dianna!
Marcy