Ugh, I know I’m old when this pops into my mind, but it’s worth a mention.
Those toy store boards can be quite expensive, especially in the long run. One way is, as mariposalabrown said, if something breaks you’re stuck buying everything over again, whereas skate shop boards allow you to replace old or broken parts. Once you have one, the upkeep cost is fairly low, and the benefits are long-term. I still have my old board, it was middle of the line in the 80s, and when I get bored I still fool around with it, and the parts are still available. Well, it’s not really my old board, everything has been replaced over the years, but you get the idea.
More importantly, your kid’s riding on it. While it’s not exactly the safest sport, using a cheap board only makes it worse. At some point, whether he sticks with it or not, he’s going to do something really, really stupid. (When I was just a little older than he is, jumping off garage roofs was a way to pass the day.) Maybe after he’s stuffed it in a closet for a couple years and his teenage friends talk about skating, and he pulls out the only board he has.
While a better board may not always be safer, they tend to be more durable, or at least absorb an impact that would wreck a cheap deck. Worse, really cheap toy bearings/wheels can fail unexpectedly, throwing him at the worst time. Saving a few bucks might cost you more in copays in the long run. Buying a better board doesn’t eliminate the problem, but it can mitigate these problems a little, simply because if his friends can do something, his board should keep up, and it won’t surprise him as easily.
If he does stick with it, a full set of pads are a good idea, too. He won’t want them, and he definitely won’t want to wear them if his friends aren’t. Buy them anyway. Stick them into a corner someplace. If he does move beyond casual sidewalk skating, and starts doing the more dangerous stuff, he might start wearing them on his own. They lack the coolness factor for some, but they allow you to fall when you need to, preventing those bad, uncontrolled falls that happen when you stay on the board too long. After a while you don’t even think about them, and you can focus on your skills instead of worrying about scrapes and bruises.
It’s probably a good idea for you to at least skim wikipedia or google a few related sites so you have an overall idea of what to look for when buying one, away from sales pressure and your own awkwardness around the subject.