The boy is desperately wanting someone to sword fight with him right now. His sister rarely participates in his barbaric battles and with no neighbor kids nearby, he finds himself alone again. One makes for an awkward battle.
The girl suggested that he fight with a tree. Apparently he has done this before and found it less than satisfying. "I don't want to fight a tree again; trees have no sense of strategy," he moaned in response.
He pleaded his case: "I want a REAL enemy, someone I can strike to the ground and hear them say 'I give up.'"
Ah, my sweet little boy.