Why is he called "Ben Zerach"?
Radak: It is because Zerach was the head of the family.
Why did Yehoshua bring Achan's children and all of Yisrael to the place?
Rashi: It was to see his punishment and chastise them, lest they do like him.
Why did Yehoshua bring Achan's ox and donkey to the place?
Rashi: It was to eradicate them, like it says "Yisaref
Why does it say that he brought them up to the valley?
Radak: There was a mountain between Machaneh Yisrael and the valley. It says "va'Ya'alu" regarding ascending the mountain.