cut them down:把它们砍倒。

cut down them这种结构不存在。

cut down这种短语动词是实义动词加副词构成的。放后面跟宾语时，如果作宾语的词是名词，那么这个作宾语的名词可以放在动词的后面，也可以放在副词的后面。如果是代词作宾语，代词不能放在副词的后面，只能放在动词的后面。

例句：

There are some trees on the hill, and a few people are cutting down the trees.

也可以说：

There are some trees on the hill, and a few people are cutting them down.