Is there anything you think they should learn about even if they don't find it interesting?
For maths, but they pick it up pretty easily in day to day conversations and activities (ipad and gaming helps, imho)
My 11 year old went to school part time, lasted half a term, but his maths skills were on par with the kids there, if not better. He had never done any formal maths.
I only speak on behalf of our family, everyone has different experiences, but it's still important to recognise that life and education can be done differently.
I was 1-2 years ahead of public school coming from private school around 6th grade (11-12yo).
Classic literature.
Controversial literature.
Philosophy.
A nonnative language, preferably Mandarin.
Phonics.
Dialectical method exercises in critical thinking.