Primary/Elementary School Maths: many children can learn on their own from here, but it's fine to work through it with them if they can't.
Upper Primary School Maths: long multiplication and division, fractions and decimals.
Different school systems can work in very different ways, so it may not be obvious where you fit in with all this. If you're 12 (or you're going to become 12 this year) you should be getting on with the stuff below (though you should work your way through all the stuff above anyway to make sure that you haven't missed out on anything). If you're younger but have done all the stuff above, just move on and do the stuff below anyway. There's no magic age at which you suddenly gain the ability to do anything in maths that you couldn't have done when you were many years younger, so just keep going.
Equation playground: months of "hard" maths compressed into a day of play.
The content for the sections below has yet to be written, so don't click on the links:-
Rearranging equations: more months of "hard" maths compressed into a few days of play.
Maths for exams: for the kinds of exams you would normally sit at ages 16 and 18.