They arrived at a farmhouse, in front of which sat a small boy.
这句话无法用where 关系副词改写 因为如果改写了 意思就不一样了。
介词加 关系代词 不一定 完全等于 关系副词
它 要比 关系副词 具体
They arrived at a farmhouse, where sat a small boy.
they arrived at a farmhouse where a small boy sat in front of it
这不是定语从句,只是where引导的状语,也不是状语从句
仅供参考