For 8 - 9 year old children, I think a game cube would be better than a PS2 (or an X-Box).
1) The pad is easier to handle (especially for little hands), whereas the PS2 and X-Box pads are bigger and heavier.
2) The games are more colored and bright.
3) The Game Cube will probably last longer than the others (personnal point of view)
4) The Game Cube is good for all ages, which means that they will probably not stop playing (or want another one) when they'll grow up (don't believe those who say that the NGC is for children: they probably have never played!

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5) The NGC is (nowadays), the most powerfull support (it's more fluid, bright, spheric, etc...)
Games ideas for children (NGC): Tarzan freeride, Donald Pecca, Rayman 3, The incredibles, Luigi's Mansion, Mario Bros Smash Melee, Mario Kart double Dash, Donkey Konga, Viewtiful Joe