<今日の進捗>
Book8 - 60 (If your boss kept nagging at you, how would you react?)

<オリジナル回答>
Supposing your three children began to shout one another down, what would you do to stop them?
Supposing my three children began to shout one another down, I would pull them apart to stop them.

【解説】
ここでのsupposingは接続詞でもし~と仮定するならという意味です。接続詞なので後にはS+Vが続きます。