Just to give you hope, I gave up music as a teenager because I did not have a grasp of the foundations. Years later it still gnawed at me. It took me 5 years to find a teacher in London. And turns out it wasn't me, it was my old music teachers that were the problem. It simply wasn't possible for me to self teach grade 5 without a detailed and thorough music education. Now I am about to take grade 5, after two and a half years of studying the "Danish" and my lowest score to date was 93% for grade 4.
So if you or your child just doesn't get it - it's not you. It's the learning style. Keep the faith. Oh, and it can be fun too. Now I'm thinking about becoming a music teacher myself...