Well, I had the same problem that you have with the music. I was pretty sure, that it was highly important, but I wasn't able to hear what it said. To my ears, the piano does sound slightly 'out of tune', but whether that is true, or if that means they use 'mean tuning', I don't know. I don't think it is meant to be music that Gyuri has made, because he seems mostly concerned with Bach, and dissapointed with the end-results.
Hm. The children. I'm pretty sure, that I thought they were imitating dictators and marching bands, in the most generic way possible. It's just noise and violence. As such, it could be seen as another instance where the film indights not so much communism as authoritarian rule in general. The children are somewhat like the mob. We don't know where they get it from, but the result is violent. Another focus on effects rather than causes. Dunno, that seems a bit obvious, and probably more inspired by me just rereading the blogposts, as I found them again. It's a bit vague to me.