When a child uses the word "cat" to refer to the family cat but not any other cats, occurs?

When a child uses the word "cat" to refer to the family cat but not any other cats, underextension" occurs.

Referring to a specific thing or item in a segregated sence yet neglecting to allude to another thing in a similar classification in that sence is a delineation of underextension. Usually utilized by youthful kids, for instance when a kid calls their teddy bear Teddy yet alludes to other teddy bears as 'toy'.