Children, especially infants are under immense amounts of pressure. An adult personality would stress out and break down under similar pressure.
The difference between adult stress and childhood stress is that the mind and personality are yet unformed and flexible - adults tend to be more rigid in their internal structure.
Stress is caused by resisting pressure one way or another. A child is not capable of greatly resisting pressure - they go with the flow. Adults live in their own minds, their own perceived realities of past, present and future. A child's mind is generally in the NOW.
The stress of an adult is caused by pressure on the conscious mind; the stress of a child is caused by pressure on the subconscious mind - and the subconscious mind is better prepared to deal with immense pressure.
When pressure/stress builds up in a child, he or she starts to cry, letting the frustration out and dealing with stress in the most direct fashion possible. When the frustration is out, the child dwells on it no more.