[Kite taps his foot against the ground.]

I can understand that, to a degree. That may be 'Slenderman's World' while this tree is his 'gate' to ours. If this tree is truly his 'gate' into this world then it would make sense to make it strong enough to withstand our attempts to destroy it when the children start to go missing. It is like my own body, in a sense. Like I can survive having my limbs removed, this tree can survive fire until all the children are captured.