Question

所有的孩子们是all children还是all the children

Answer 1

all children是泛指"所有的孩子" ,没有指定具体范围的孩子。

all the children = all of the children = all of these/those children

是特指"这批孩子中的所有人",一般是面向一个特定的群体. 

eg:

All children like ice-cream.

孩子们都喜欢冰淇淋(喜欢冰淇淋是所有孩子的天性,不论肤色与地域如何)

All the children are in the classroom.

(这个班的)所有孩子都在教室里.

Answer 2

得看前后文，如果单单是全部小孩
All children should come to school next Monday.
但如果有指定
All the children who have question should ask their teachers.