。
Be a good kid while I'm away.
Literal
Absence-during good child [with-で] be [command-んですよ].
いい子でいるんですよ is a warm parental instruction: いい子 (good child) + でいる (be/stay) + んです (explanatory) + よ (emphasis). The combination of んです + よ softens the command into caring advice. A classic thing parents say when leaving children.