Stuff I've enjoyed massively on the DS:
Advance Wars: Dual Strike and Dark Conflict (just say goodbye to any free time, ever)
Lego Star Wars (just getting through is easy, and thus great for little kids - actually completing everything is a huge task)
Age of Empires: Mythologies (a few frustrating levels, but suprisingly engaging unit animations and interesting strategy)
Space Invaders Extreme (don't knock it 'til you've tried it - it's crazy stuff)
Professor Layton (probably shouldn't work, but boy does it ever suck you in).
I actually think the DS is one of the most remarkable platforms I've ever had access to. The fact that my boy has been loving it from the age of 4, as well as my mother at 68, and me at 40 really says something - and that the same basic black unit has served all three of us without complaint for several years now.