Wargames for a 9 Year Old

One thing you might try is a wargame actually designed for children, Crown Tactics' High Ground. The game is quite abstract (it's very loosely based on 18th-century warfare) and features a symmetrical setup, and you can move and/or fire only one or two units per turn, so in some ways it's like chess, but it uses dice to resolve combat, meaning that the strategy is a lot closer to wargaming than to chess or similar games.

It's also a very pretty game, with big plastic pieces (squares for infantry, rectangles for cavalry, and triangles for artillery) that have little pegs (sort of like the game Life) that are removed to mark combat damage.