Final answer:
Significant injuries in infants and children can result from physical abuse like shaken-baby syndrome, accidental falls, or vehicle collisions when unrestrained.
Step-by-step explanation:
For infants and children, significant mechanisms of injury include physical abuse, which may manifest as beating, kicking, or even in severe cases such as shaken-baby syndrome resulting in brain swelling and retinal hemorrhage. Another mechanism is a fall, where even a relatively short fall of 3 feet can result in injury when a child is accidentally dropped. Lastly, injuries can occur from vehicle collisions, especially when the child is unrestrained, leading to serious harm or fatality due to their vulnerability.