Final answer:
The girl is exhibiting signs of regression, which is a defense mechanism where an individual acts younger than their age.
Step-by-step explanation:
The girl is exhibiting signs of regression. Regression is a defense mechanism in which an individual acts younger than their age, often in response to stress or anxiety. In this case, the 7-year-old girl sucking her thumb and changing her speech patterns to those of a toddler are behaviors typically associated with younger children.