If I'm reading this right, you don't really want different add ons for different accounts. You want different add ons for two people using the same account. You want one set when you're playing and another when you let your 9-year-old log in to your account to goof around.
I suggest checking out
Addon Control Panel. Among other things it allows you to save various add on configurations. So you could save your standard set as one configuration and then save another configuration with just the add ons that your 9-year-old needs. Then you can just load whichever configuration you need when you need it.