To answer your questions, Yes it gets full but not crowded (hope that makes sense), they do have (what I call) an organized chaos. It gets loud, its a kids club not the library or school. BUT there are (what my girls called) "quiet areas". Here is my experience:
My daughters were 3 and 6 last year when we went on our cruise. My oldest daughter has a sensory processing disorder which can be set off when around loud and or busy/moving areas. This is not our first
DCL, but it was our first with the kids club. I wanted my girls to experience the kids club, and wanted to give us a break for a bit to relax while on the cruise.
My oldest daughter enjoyed it! She said that there were moments (depending on the activities) that became too loud and too "hectic". During these hectic times she would move herself to quiet areas to do other activities (her way of decompressing). When the kids are ready to leave for whatever reason, the parents are messaged/called. My oldest daughter never asked to leave, but I was called a few times for my youngest.
The cast members are very receptive to the kids' emotions and can easily spot when a kid is uncomfortable. The first time I was called for my youngest, a cast member noticed her wandering around. when a cast member approached her, my daughter said she was ready to leave. They called me, and when I arrived she had changed her mind and wanted to stay (ugh!). Later she told me she couldnt decided where to go next. The other times I was called to pick her up was when she was ready to go to the pool or wanted "eye-scream". She said she loved it but preferred not to spend all day in the kids club like my eldest.
I was very surprised with the lengths of time my oldest was at the kids club. She was the one I was worried about. In the end, she was never overwhelmed or in any pain with the loudness or business that goes on in there. When I asked for their feedback, my oldest said she felt like she was really on vacation, not like at when we go to Disney World. She said she had time to relax and time to play. My youngest said she like the kids club too because she felt like she could come and go as she pleased.
In the end, I cannot speak for your kids. They dont have to mingle with other kids if they dont want to, they can keep to themselves, and/or they can hang out with eachother. If they dont want to do one activity, they can do another-or none at all! If one area seems too much, they can move on to the next. I dont understand how Disney or these cast members do it!
How did they figure out a way to let kids be kids, yet have everything under such control? I just don't get it.
Hope my experience, the feedback my girls gave and my thoughts help answer your questions.