What a nine-year-old is working on
Fractions of a quantity, which is the useful half of fractions and the half that makes the rest possible. The order of operations, where brackets change an answer and the rule is a convention rather than a fact about quantity. Two-step word problems. And the spelling rules that have a reason behind them, rather than lists to be memorised.
Being ahead, and being bored
A nine-year-old who already knows the material is held up by a game that never lets a wrong answer pass. That is what the strictness setting in the Parents Area is for: on Brainiac one mistake is forgiven and the round moves on, hints are switched off, and a wrong answer costs points. It turns practice into a test, which is what a child who is ahead actually wants.
Chess, properly
Nine is a good age to start playing whole games rather than solving positions. The computer here has three strengths and the weakest is meant to be beaten; the point is to play enough games that the pieces stop needing to be thought about.