If your son/daughter is actively sailing a C420 for several years, you might find it a good idea to give him/her access (open as to how) to something more fun and exciting as well.If my son/daughter said to me “Dad, I am going to quit sailing. I find the C420 does not meet or exceed my performance requirements”, my response would not be nurturing.
If the club/coach isn't doing it, and you have a friend who owns some other boat -Something fun, exciting, or just different – get them on that.
The "dead end" road seems to be clubs / coaches / families that have kids sail the same boat for years and years. The kid might not be able to articulate "this has gotten boring" but they'll slot it in a mental box of something that's kind of stuck because they are kind of stuck. So when it's time to move on, they'll move on.