Yes. I think you can bypass the idea of "frequency domain", and focus on regular / fast changing. I think a smart 12 year old would understand the intuition tha

Yes. I think you can bypass the idea of "frequency domain", and focus on regular / fast changing. I think a smart 12 year old would understand the intuition that fast changing and high frequencies are somehow related, I mean physics gives plenty of concrete example. And then you explain that for images, contours are fast changing, and you have the justification for Fourier coefficients truncation ~ compression. Explaining how DCT helps instead of DFT is the harder part.