There are thousands of games for the DS. I can understand how daunting it is to select a suitable game for your children. My girls are now 14 and 17 and have been playing the gameboy and ds for more than 10 years.
My two girls enjoyed the following games:-
The Legend Of Zelda: Phantom Hourglass action
Mario Kart DS action,racing/driving
Pokemon Diamond
Super Mario 64 DS action,adventure
Animal Crossing: Wild World adventure,rpg,simulation,strategy
Pokemon Diamond/Pearl rpg
Brain Age 2
The Sims 2
Brain Age: Train Your Brain In Minutes A Day action,strategy
The Urbz: Sims In The City action,adventure,simulation,strategy
SimCity DS action,simulation,strategy
10 yo girl would like
Animal Crossing: Wild World adventure,rpg,simulation,strategy
The Urbz: Sims In The City action,adventure,simulation,strategy
SimCity DS action,simulation,strategy
7 yo boy would like
Mario Kart DS action,racing/driving
Pokemon Diamond
If you are stuck for ideas you can also look at the top rated games
e.g.
TOP DS Games
The highest rated games can be a bit difficult/challenging but they can also grow into them as their skills grow. As a suggestion try renting from a video store before you buy.
Good luck.