I love the description of this festival - what a superb idea! My dcs are 6, 4 and 1, so we're at a fairly early stage in this process, but its something I think about a lot!
To answer the question, I need to explain my own experience: I was brought up listening to a huge range of music. The joy of growing up in London was being exposed to a huge range of music events (particularly Sunday orchestral perfomances on the South Bank and at the Barbican, although I never enjoyed the violin concertos), as well as my dad's massive collection of Motown, the Beatles and Bob Marley. My dad encouraged me to trust my own tastes and reject the anodyne SAW sound of the mid/late 80s. I skipped all pop music, and went from Motown to 60s hits, 70s prog rock, heavy metal, industrial rock and into techno, dance, jazz, blues and all over the place in between over the years. Music has been a huge influence and factor in my life choices and I'm determined to give my dcs the same appreciation. My dh comes from a totally uncultured different background, and only really listened to pop when we met. I've been training him slowly 
Anyway, I'm determined that exposing my dcs to an enormous range of interesting, intelligent and well contructed music (Handel, Back, Beethoven, Mozart, Shostakovich, Tchaikovsky, Pink Floyd, The Doors, Metallica, Rolling Stones, Orbital, The Orb, Eat Static, Ozric Tentacles, Joss Stone, Amy Winehouse, Rachmaninov.....you get the picture) is the best way to allow them to develop and grow their own tastes and ideas. I want them to feel comfortable expressing an opinion about music, and love that they choose to put on all sorts of music on the cd player in their room. I recently bought them Peter and the Wolf and Peer Gynt, and we've had some great discussions about different parts of these pieces make them feel.
Overall, I cannot sufficiently express how important I believe it is to expose my dcs to a massive range of music :) :) I was always determined to avoid the bane of listening to childrens nusery rhymes in the car, and reckoned if I got them started early enough on music I like, that could be avoided :) :)
DS is now 6 and keen to learn guitar or piano, and we have agreed he can start lessons next year. His singing voice isn't great, but singing gives him pleasure (I can't sing a note, but also love it) so we're happy to encourage that whilst giving him the 'tools' to continue to be musical once someone points out to him that singing isn't his biggest strength!!
The last classical concert we attended in full was the ENO production of The Nutcracker ballet at Christmas. This was also the dcs first ballet and they sat utterly entranced through the entire performance - but I only had the confidence to take them as it was one of the 'family' performances. I'd love to take them to more, and am planning to take them to some outdoor performances this summer. I would love to take them to more performances but always worry that someone else will have paid lots of money for their ticket, and will be irritated by my 1yo getting fidgety, or 4yo whispering - so we will be sticking to 'family friendly' events for now.
As they get older, I fully intend to drag them to art galleries, museums, NHS properties and classical concerts every free Sunday as its one of the things I think my parents got really 'right' for me :)