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.