How safe is v-brake system with handbrake and coaster brake for child going downhill?

This brake setup would probably be the safest for a five year old, which is why its so popular. It would be rare for a child of five to be able to brake hard enough and go over the bars with V brakes.

V brake front + Coaster brakes are common on children bikes because children generally do not have the hand strength to operate brakes properly, but you need two brakes and it helps them learn to use hand operated brakes while still having one that is effective. Expensive hydraulic disk brakes would be effective, but too expensive to be practical on kids bikes, and risks over the bars and front wheel loss of traction type accidents far more than weaker brakes.

One big advantage of coaster brake is it is nearly impossible to go over the bars, yet possible to lock the rear wheel (need for new tires makes it expensive with some kids). To get the brake to work, you have to press back on the pedal, transferring weight to the rear. Any weight shift forward, needed for a over the bars, release the brake.

Can anyone explain to me how the braking system works on this model?

Well, a coaster brake works by, instead of allowing backwards-pedalling the freehub at any speed, using that motion to engage a small drum brake in the rear hub. Because you're using the pedals' leverage, it's actually quite easy to lock up the rear wheel this way, which children will often take great excitement of drifting around corners. (I certainly did.) The flip side, apart from tyre wear, is that the habit of braking mainly with the coaster brake won't teach actual emergency braking. Though it's possible to get quite good stopping out of a rear brake only, this requires putting your weight way over the back (i.e. move your body behind and below the saddle). In normal middle position, you can get only about ½ of the maximum front braking power from a rear wheel before it locks up and doesn't do anything more.

So that's probably the biggest danger: that your child learns himself to rely too much on the coaster brake, and then in an emergency won't have the feeling for how to safely use the front brake – and that will send him over the bars. (This did happen to me, though I must have been at least 8 by then.)

So, I would indeed recommend a bike with two hand brakes over one with coaster brake. However, a coaster brake per se is actually safe for children. (It's unsafe for adults who have gotten used to being able to pedal backwards at any time, but that's another story.) It should be fine for your son to use this bike, just make sure he's aware of the limitations of the coaster brake. A good way to teach proper brake feeling is to let him have a go at some actual technical “mountainbike” descents, at least 40% incline and soft ground, because that's where the rear brake just won't be enough anymore but a crash won't cause any harm. (Still, of course, start small with this.)

Going over the bars with V brakes is only possible given these circumstances:

1. A frame where the V brake is mounted unusually high, so that the leverage of the V brake arms is larger than usual
2. A brake lever with unusually high mechanical advantage
3. A rider of average weight (or less)
4. A foolish enough rider to not realize that when the rear wheel rises, the brake must be released

For children's bikes, their hands are smaller so the mechanical advantage of kid levers is probably lower than for adult levers. However, children are lighterweight than adults. I assume these two effects somewhat cancel each other out.

I used to have a bike with unusually high mounted V brake arms. I changed it to drop bars and chose Cane Creek drop bar V brake levers (that have a slightly larger than usual mechanical advantage, especially when riding on the drops, and the old flat bar levers had far lower mechanical advantage that the drop bar replaced). When I had this bike, I had average weight. Then going fast downhill, I avoided tree branches and suddenly noticed I had gone too far left and there was an oncoming cyclist. I braked hard on the front brake having been used to the weak flat bar levers -- only to notice the rear wheel rose with the new drop bar levers. Then I immediately released the brakes and realized that the sudden braking had removed enough of my speed that avoiding the oncoming cyclist was possible and did just that.

It's rather unlikely that the specific bike for your son would have both (1) and (2) properties at the same time. Even if it has, I assume most people react to rear wheel suddenly rising by releasing the brake lever. So I don't really think the "you'll fly over the bars" is a problem. Besides, my not-going-over-the-bars-but-raising-the-rear-wheel event happened only because I had become used to weak levers and replaced them with strong levers. For someone who is used to the current brake levers, it's possible to regulate the braking force. I rode after this many years with the drop bar levers, never again raising the rear wheel because I knew how much braking makes sense.

A far more likely event is that your son rides down a steep gravel road and brakes with the front brake, locking the front wheel and causing him to crash.

And for someone who thinks that other style of brakes like cantilever or sidepull would be better, all current rim brakes have about the same mechanical advantage. I also have a cantilever braked bike and a 160mm hydraulic disc braked bike, and both of them have equally powerful braking when dry (but the disc brake bike has immediate strong braking when wet whereas the rim brake bike has brake delay when wet). So no matter whether you choose rim or disc brakes, all of them are equally powerful.

So, in summary: you safely regulate the downhill speed by knowing how hard you can brake. In some cases (very heavy rider, low mechanical advantage brake system, weak hands) it's impossible to brake too hard on pavement downhill. The greatest danger is using the front brake on gravel or dirt downhill so hard that the front wheel skids, causing a crash. The "you'll fly over the bars" concern is not valid.