This is where you really have to know how mature you kid is. I got my first hobby grade r/c for Christmas when I was 8. It was a kit and my father and I spent the next few days building the kit. We took the electronics out of his old Tamiya Rough Rider to get it going. All in all it was probably a couple weeks later before the car was all ready to run. I still remember the last thing that needed done was to paint the body. We laid down several coats of Daytona Yellow, then headed off to grandma's house to keep my mind occupied while it dried. Later that day I got to run my Christmas gift for the first time.
For Christmas when I was 13 my dad got me another kit. This time I wanted nicer stuff electronics wise because it was a car just for the track. So I got the kit for Christmas, and again we spent the next couple days putting it together. It took until my birthday in March of scrimping and saving every bit of money I could to get all the stuff together to have the car ready to hit the track. The deal with my dad always stood, whatever money I could raise, he would match it.
A kit not only taught me how to work on the car, but patients too. Now when I get a new car I don't slam it together Friday night, paint a stupid one color body, just to race it Saturday and have the thing handle like crap and fall apart. Instead I show some self control. I show up a couple weeks later with a car that is perfectly built, painted with my paint scheme, a fresh set of the right tires for the track, and a car that works well and stays together.
If your kid is mature and mechanically inclined a kit might be a good option, if he/she is more of the run and gun type an rtr will probably make them happier.
Either way with you guys being new I would not recommend anything used. It will be more of a hassle than the small amount of money you might save. Not to mention giving a used thing as a gift. It would be one thing to get a used car, fix it up, then give it to him. But at this point in time you don't have enough time to do that. If you want to see that look on his face a new kit or rtr is the best way to go.
Personally, I would recommended a Traxxas 2wd Slash. Big, durable, waterproof, and you can get parts just about anywhere. I would wait on the brushless as an upgrade for his birthday. I also don't know much about the Exceed stuff. Personally I would stick with a Traxxas for a new venture in this hobby.