いいか。店内では大人しくしてるんだぞ。

。。

Listen. You're going to behave yourself inside the store, got it?

Literal

Listen. Store-inside [at-では] obedient doing [progressive-てるんだぞ].

いいか 'listen / got it?' is a masculine attention-getting opener for issuing orders. 大人しくしてる is the casual progressive of 大人しくする 'to behave / be quiet.' んだぞ stacks the explanatory んだ with the masculine emphatic ぞ — 'you ARE going to be (behaving), understand?' Parental/authority-figure speech.