Hello! My eldest daughter is 10 and I have to say her nintendo DS is probably THE best value for money christmas gift I've ever bought in her life in terms of play hours!
At present her 2 games she constantly goes back to are "harvest moon" - where you buy and manage your own little farm whilst you form and maintain relationships with other people in the village and save the princess by saving the harvest sprites (her words- I just asked her

) and "Animal crossing" - where you have a little person, build their home, grow plants, go to work, send notes to people in the village, buy a bigger home... pretty much a virtual life from what she is telling me. Both have great tutor modes on them where you follow online instructions to learn the basic skills. Nintendogs she liked but grew quite bored of quickly. Cooking moma was/is fun but i think would get frustrating when first learning game play.
Animal crossing also has the biiiig advantage that you (player) can link up with other friends who have a ds and that game and your person can visit their town (my dd1 plays with her bestfriend, my mother, my sister)
Hope that helps a little.
Accessory wise I'd say buy an airformed case to store it in, protective film for both the screens and some spare stylus pens. If you travel alot a in-car charger is also a good investment.