a lot of people will say it isn't worth the upgrade, especially for younger kids. the dsi basically has a camera (low resolution grade), ability to access the internet, a tiny bit bigger screen, much better sound quality (and the ability to play music from a SD card). the new one can't play the old GBA games.
i was able to sell my ds on ebay plus a case and 3 GBA games for $135. So to me, it is worth it to upgrade. (assuming the post office ever marks the package as delivered....)