In our experience, kids have a great time with this sort of thing. They often grasp the idea much quicker than adults - after all, playing 'let's pretend' isn't as alien to them.
If you're still worried, I suggest that you let your daughter have a look at one of our free extra characters (from another game). If she sounds enthusiastic after reading that, then you don't have anything to worry about.
Here's the link to the extra characters: http://www.freeformgames.com/extra_characters.php
(If you're throwing the party as a surprise for your daughter, then you might try showing the character to someone else instead.)
As for Les and the hypnotism, I suspect that whomever is playing Les will come up with all sorts of ideas without any help...
I hope that helps - and I hope you have a great party!