According to Chess and Thomas' temperament classification system, a (n) child is generally in a positive mood, quickly establishes regular routines in infancy, and adapts readily to new experiences.

Easy child

Explanation:

There are three category of children, according the psychiatrists Thomas and Chess, The Easy Child, The Difficult Child and The Slow to Warm Up Child.

The easy child - as term suggest is one of best type of child, who have positive mood, very regular and adapt to new conditions very fast. The Difficult Child - these of type children, cry most often, irregular daily activities and low to adapt new conditions. The Slow to Warm Up Child - these type of children, have very low activity level, least to adapt new conditions.

Hence, from the given information of the que4stion,

The correct term is easy child.