Don't grow bigger with 〜てはだめだ
Grammar Explanation: Must not do with 〜てはだめだ
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:
- 「チノちゃんは大きくなっちゃだめ！」
- "You mustn't become bigger."
Note that だ has been dropped from the expression here.