most of the new games are being released for the 3DS. You can turn off and parent protect the 3D function so it doesnt get used - both mine (age 6 and 9) have had the 3DS for a couple of years but never use the 3D function.
Regular Ds you'll only find 2nd hand (ebid etc.) Wouldn't your budget stretch to a 2nd hand DSi (maybe £70 on Ebay if not too close to Xmas)? I wouldn't want my 4yo to have one, tbh. Mine wouldn't appreciate how breakable it is. Now 5yo, DS has never asked about having own DSi, either.