Don’t grow bigger with ~てはだめだ


Grammar Explanation

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On the topic of Chino outgrowing Cocoa, Cocoa is fine with it until she learns Chino won’t be like a doting big sister to her.

ココア:
「チノちゃんは(おお)きくなっちゃだめ!」
“You mustn’t become bigger.”

Note that だ has been dropped from the expression here.