According to Newton's Third Law of Motion, the snowball hits the boy just as hard as the boy hits the snowball. So, why does the snowball get smashed while the boy remains relatively unhurt?
First of all, the snowball is made of snow, which generally breaks apart very easily, but in order to totally understand why the impact affects the snowball so much more than the kid, we must use Newton's Second Law of Motion. According to Newton's Second Law of Motion, if forces of the same size are exerted on objects of different masses, the object with the smaller mass will accelerate more. The snowball has a MUCH smaller mass than the boy and so is accelerated A LOT more than the boy. In fact, the left side of the snowball is accelerated through its right side!
Even though forces which are equal in size are exerted on the boy and the snowball, the snowball accelerates a lot more than the boy because Newton's Second Law of Motion says that if equal-sized forces are exerted on objects with different masses, the obect with the smaller mass will have a larger acceleration.