Just a thought - last year my boys each got Nintendo DS's for Christmas too. They were 5 at the time. Here's what we have discovered:
Protective case/cover - go with the Nerf cover. Dropping the DS can easily break a hinge. The DS is usually still playable, but it's $50 (+$10 shipping) to send off and get repaired. (When mommy tries to do it herself the DS completely quits working.

)
To carry the gear - we found a netbook case (small laptop case) on clearance at Target for a couple of dollars last year. They were able to store their DS, headphones, chargers, games, and extra styluses.
To store the extras - we have an old zippered lunch bag that we keep the original game boxes, extra styluses, screen protectors, ect.. (A regualr zippered lunch bag would also be a great everyday storage bag too!)
The extra accessories - I got protective cases, screen protectors, car chargers, and styluses off of eBay for cheap. (I think I got 50 styluses for 3 dollars. Read the feedback and order the cheap ones. They may take a bit longer, so buy them before you ned them.)
Have fun - and don't be surprised if you LOVE it too!