OK... I just wanted to chime in on the ipod touch Vs DSi.
Ipods games are free, but they are mostly simple "on the pot" time wasting kind of games. You don't get the gaming games on an ipod like you do on a gaming platform such as the DS, PSP, etc.
My oldest (9) has both because she won the ipod. They get about equal playing time. My DS8 has access to play my ipod, but prefers his DSi XL and will play one game non-stop until he beats a game. Or at least beats the level he is on.
We have 2 DSi XLs and 1 3DS. I can't really say which is superior. My DS8 really loves his 3DS, again, because of the advanced games. We drove from California to Northern Idaho for spring break, and I literally didn't hear a word out of him in the car because he was working on the new Kid Icarus game!
The main reason I like the XLs is because with the standard DSi or the 3DS they hold it so close to their faces... not a problem with the XL. It also seems like it is easier to hold. The bad is since the 3DS came out, there are even less XL accessories in stores, and in general.
As far as games, my DD6 likes the animal type games- the Nintendogs and Pets Nursery types. Where you adopt animals and name them and care for them. When DD9 was that age, she preferred the Mario games- Mario Party, Mario Kart, and the
Lego games- Lego POTC, Lego Rock Band, etc.
Good luck and have fun!
Edited to add: Don't forget the warranty! DS is already on his third! And most places you can buy a DS will sell you a warranty that covers accidental damage. It happens!