I have a system I've evolved over the years, and while it's not perfect, it's the only one that works for me currently. It's far too detailed for your needs, but it could give you some ideas on how to organize your own based on your own needs. Here's how it works:
1) First of all, if you are using any typical genre names like rock, blues, pop, jazz...etc, do not use them as is. What I do is add a suffix, something simple like a dash or underscore, or whatever you prefer, as long as it's easy to see and not too distracting. The reason why this is important is because too many audio files are tagged by people who neither have a correct understanding of genre distinctions, nor do they share your idea of what the parameters of specific genres are. So for example, if you have tracks you want to designate as rock, and you bought a new album where the people tagging it somehow thinks that some dance pop artist is "rock," then you'll have tracks that shouldn't be considered rock among your rock tracks, and that's annoying. With a suffix in place, you can immediately tell which tracks were manually tagged by you, passing YOUR personal criteria for what you think rock music should be, not what other people think it is. This way, in the future, whenever you see genres without a suffix in the tag, you know they're tagged by someone else and not you personally, and are to be totally trusted to be accurate in their genre tagging--until you confirm the tagging by retagging them with the suffix.
2) I tend to be more detail-oriented about genre distinctions because it helps me find tracks much faster, and also because I'm somewhat obsessive compulsive about it. I'm one of those people who have all my DVD's in alphabetical order, divided by genres, and sometimes by directors too. My CD's are the same way, as well as all my books. My hard drives are also meticulously organized. It really makes finding stuff much easier, but that also means you have to take the time to do the organization. It's one of those cases where once you start, it becomes second nature, and the rest is just adhering to the system you've created whenever you have new stuff to add to the collection.
3)My parent/child genre list currently consists of these:
Alt (short for alternative, which by today's standards means more like indie or anything non-mainstream)
Alt-Contemporary (means adult contemporary, folk, or general singer/songwriter types like Sarah McLachlan, Tori Amos...etc)
Alt-Ethereal (Shoegaze, dreampop stuff like Cocteau Twins, The Sundays...etc)
Alt-Rock (Guitar oriented bands with a rock root like Veruca Salt, Bush, Peal Jam...etc)
Alt-Pop (More poppy stuff like Duran Duran, Rialto, ABC)
Alt-Synth Pop (Depeche Mode, New Order...etc)
Alt-Trip Hop
Audio Books
Avant-Garde
Big Band & Swing
Chinese
Chinese-Other (I tend to use "other for anything that's not strictly pop or rock when it comes to languages other than English.)
Chinese-Pop
Chinese-Rock
Chillout (lounge, Cafe Del Mar type stuff)
Classical (not the classical period, but the mainstream use of the word to mean all concert orchestral and "serious" music like concertos, quartets, non-mainstream piano solos...etc.)
Compilations (stuff my friends compiled)
Demos (demo tracks used for pro audio product demonstrations--from synthesizers, guitar/bass effects, sample libraries, drum machine, to anything else.)
Demo-Brass
Demo-Drums
Demo-Effects
Demo-GM (General MIDI)
Demo-Orchestral
Demo-Soft Studios (such as Reason, Orion, FL Studio, or DAW sequencers in genreal like Cubase, Logic, Sonar...etc)
Demo-Synth
Demo-Voice
Demo-Ethnic
(The actual folders further distinguishes between software and hardware demos)
Electronic (General electronic artists not strongly affiliated with a genre or jumps around between genres)
Electronic-Ambient
Electronic-Drum'N'Bass
Electronic-Electro
Electronic-IDM
Electronic-Noise
Electronic-Pop
Electronic-Progressive
Electronica
(I avoid certain types of electronic music, such as happy hardcore or other unmusical, repetitive and annoying sub-genres I can't stand.)
Folk
Funk
Fusion
Hip-Hop
Hip-Hop-Beatbox
Hip-Hop-Rap
Humor
I/G (Industrial/Goth)
I/G-Darkwave
I/G-Goth
I/G-Industrial
Instructional (audio production, musicianship, instruments)
Instructional-Production
Instructional-Bass
Instructional-Drums
Instructional-Guitar
Instructional-Keyboard
Instructional-Music Theory
Instructional-Orchestration
Instrumentalists (musicians known for specific instruments, such as Jaco Pastorious, Steve Vai, Buckethead, Toots Thielemans...etc.)
Japanese
J-Alternative
J-Other
J-Pop
J-Rock
Jazz
Jazz-Acid
Jazz-Bossa Nova
Jazz-Classic
Jazz-Contemporary
Jazz-Fusion
Jazz-Orchestras
Jazz-Vocal
Korean
K-Indie
New Age
Other Ppl's Sheit (music by fellow musicians/composers on various forums. They haven't made it big yet, so they are not categorized into the genre categories. Makes it easier for me to find them in one folder)
Pop (this includes diverse sub-genres like New Jack Swing, RnB, Dance...etc)
Rock
Rock-Classic
Rock-Hard
Rock-Instrumental
Rock-Metal
Rock-Progressive
Rock-Punk
Rockabilly
Soundtrack
Soundtrack-Anime
Soundtrack-Film/TV
Soundtrack-Games
Soundtrack-J-Film/TV (Japanese TV/Movie scores)
Soundtrack-
World
I don't listen to country, polka, or whatever other obvious genres that are missing. Even if I do, I place them in a MISC folder and call it the day. No need to go sort them out since they are not important enough to me.
Sometimes, I have further organization inside of the folders themselves instead of using just genre taggings. For example, although Soundtrack-Film/TV is all-encompassing of all soundtracks for film and TV, I actually have more detailed folder organizations such as by composer names or by only title of the film/show (if the score is not by just one composer, or are song compilations).