TBH, I think the local junior school did a pretty good job with our eldest and it's the secondary school that concerns us more.
Having said that, if you have the time and committment then good luck to you. Apart from the three R's, I think some of the important things to teach them are :
1. Be able to motivate themselves - much better to 'want' to learn than to be 'led' to the books. As I said in the school thread, I had very little self-discipline - probably still dont
2. Be able to find things out for themselves - ask questions, don't just accept facts.
3. Enjoy learning.
HTH