I see a few posts in the forum with people asking if this can be done, but do not see it as an actual suggestion in the “Got an idea?” forum.
When a person subscribes to the family plan, they should have some settings they can control for any member on the plan - whether it is a kids account or not. There are times when you want a teen to have access to some songs, but not all of them and certainly more than the kids account. Right now there is no way to restrict content for that sub account (which you can do on other services).
It would be really great if you could go a step beyond the other services and give a few options in parental controls (1) Do Not allow explicit content (2) Restrict certain songs (3) Restrict certain artists.