my kids are 16, 13, and 8 and are perfectly content with the 1 and 2 gig mp3 players. My son got one when he was 7 and was around 30.00 and he is perfectly content with his. The 1 gig holds 25 songs. We download them for him but he is able to use it just fine.
My oldest got a 4 gig now but is still deciding whether he wants to spend the money for the ipod as he isn't sure it is worth the extra money when he can plug his into the car or his stereo and is just as content with the cheaper ones and it charges by plugging into any usb device.
I would say for a 6 year old that any of them would be good. We started with the ones that take 1 AA battery and now they have moved up to the rechargable mp3 players that charge by usb port on the computer. I would recommend the rechargeable as I had to constantly keep batteries on hand.