This was not just an evacuation drill for the children, but also for the parents who practiced picking up their children.
ご質問ありがとうございます。
"This was not just an evacuation drill for the children,"＝「これは単に子供たち用の避難訓練だけではなく、」
"but also for the parents who practiced picking up their children."＝「子供たちを迎えに来る親たちのためでもあった。」
ご参考になると幸いです。
アメリカ合衆国 
