I recommend the 3ds. You can turn the 3d off, and once you do that the screen looks identical to a DSi. The XL does have a larger screen however the picture isn't as sharp or clear as it is on the DS or 3ds because of the resolution. The 3ds uses both 3ds and DS/DSi games, which means you never have to buy a 3ds game at all if you don't want to, but you do have that option and since more and more new games will only be released for the 3ds I think that's a good thing. It has a camera just like the DSi does and can do the same things, plus it has 3d if you choose to use it.
The only advantage of a DS over a 3DS is that the DS can play Gameboy Advance games (in single player mode, not multiplayer) and there is a huge library of those available in used game stores everywhere. The DSi does not have a GBA slot, so it doesn't have that advantage.
I know one previous poster suggested going with an iPod Touch instead. I would not recommend that, unless the child only wants something to play games on and doesn't really care what kind of games they play. The iPod touch does have lots of cute games that appeal to kids, plus it can play music and video. It's a great product, but it is not a substitute for a Nintendo DS, DSi or 3DS. They are completely different, and the type of games they play are very different. The iPod can play great casual games, but many of the Nintendo games are much more involved and substantial than anything available in the app store. If someone wants a DS because of the specific games available or because they want to play multiplayer games with friends who have them, the iPod would not be a good choice at all.
)I wouldn't suggest the 3-d one for a 7 year old specifically due to the warning that children under 9 should not use the 3-d -- would be a waste.
