。。
Be quiet, or you'll get scolded.
Literal
Be-silent-stay-[command-なさい]. Otherwise, be-scolded [emphatic-よ].
黙っていなさい is ~ている + ~なさい — 'be in the state of being silent.' The contrast with 黙りなさい (also possible) is subtle: the ている emphasises remaining silent over a stretch of time, not just falling silent at one moment. Standard parental admonishment.