I'm back from Tennessee and here's how my DS went over with my little niece, five, and nephew, two:
MillionHeir Mystery Case Files: Mixed results. We searched for hidden objects together. They liked pointing things out. But they couldn't play this one on their own.
Madagascar 2: Poor results. My niece knows the characters. My nephew thought the game looked funny. But they couldn't figure out what to do in this 3D platformer and didn't ask to play it a second time.
Animal Crossing: Wild World: Success. They loved this one. Specifically, they loved just walking around it, showing a preference for d-pad movement over stylus control. My niece thought fishing was incredibly fun.
Kirby Super Star Saga Ultra: Failure. I told them I had a Kirby game. They didn't care.
Electroplankton: Success. This light-and-sound music toy/game entranced my niece like almost nothing else. She played all the modes and thinks they're delightful. That this game was designed to be learned without text instruction probably made it the easiest for her to understand. She was lost in the game for several minutes at a time this weekend, merrily playing in her own world.
Next: I'll get back to playing more games for my own tonight. I hope the "Resistance 2" campaign isn't as much a disappointment as some reviews suggest.