He is an adult. Maybe it doesn't work with children?
I used it once with the sleeping potion. My Sim just mixed it up like that, no bar was required. But he drank it himself, that's why I'm not sure it's really helpful for pranks.
For the Imaginary Friend, if the child has the potion in its inventory and a high enough relation with its IF (best friend), the option will appear on the social interaction menu. I think it's on an orange background, same as opportunities' special interactions. When the relationship is not good enough, the option is there but greyed out.
This is all I've found out so far. But my adult potion maker is not a rebellious Sim either :-)
A last thing: the Radical Reparum lasts 1 hour but makes any repair super fast. Nice too when you want your Sims to something else than unclog the toilet.