Blood And Fire.

Father just shrugged and said, "I don't want to fight. I just want to help them make a decision. I already eliminated the weak one, so I won't kill anymore. I promise." 

Mother asked him, "Will you save them if they go with you and encounter life-threatening danger in a fight?" 

Father snorted. "Of course not. What's a fight without danger? If they go to fight, then they must be prepared for the consequences."

Mother said to them, "Do you hear that, kids? If you go with him, then you must be prepared to die in the fight." 

That is information that they didn't know earlier. They assumed that their father was taking them for supervised hands-on training and not a fight to the death. It made the prospect of going with Father unpalatable. Even First shrank back just thinking about it. It reminded her of the ordeal she just went through. 

Father didn't seem to notice their grave expressions. He asked with enthusiasm, "So who wants to fight?"