Who wouldn't like Pokemon? In Pokemon Black and White, you get to build up your own team of virtual pet-type creatures, who help you battle other Pokemon, and grow stronger as they do so, as you set out on your quest to become the Pokemon Champion - the best Pokemon trainer in the region.
The emphasis in Pokemon is on taking care of your creatures, friendship and loyalty. The battles are never violent and your Pokemon never die - they simply faint and can't battle again until you get them healed at a Pokemon Centre, where they'll be back good as new. It's a whole new world to explore and lose yourself in, with cool creatures, a simple, but deep battling system, and an epic story to tie everything together.
The one minor downside of the story, however, is that there is quite a lot of reading involved, which is essential to understanding the game - so your child will need to be quite confident at reading to fully enjoy Pokemon. That said, this is a game that they'll enjoy so much and actively want to play, that it may well have the added side-effect of helping your child improve their reading.
With nothing in the way of sex or swearing, and only very, very mild, cartoon violence, there's very little in the way of mature content in Pokemon Black and Pokemon White. While the majority of the game involves battling your Pokemon against other teams, things aren't actually all that violent, with your Pokemon simply shaking a bit when they perform an attack, and your opponent flashing when they get hit. With no physical contact actually made, this is a massive hit with the kids, and good family friendly fun.