get next or previous post link

Hi all,
I have a problem related to next and previous post link.
I created one parent category is “All”. Then i created 3 child categories : child1, child2, child3.
When i add a new post for child1, i also check parent category “All”. Do the same thing with category : child2, child3.
But when display post, i just want to get the next and previous link in child category, without parent category.
So, I use the bellow function:
next_post_link('%link', __('%title »', 'ipin'),true,'1');
1 is parent category ID.
But it’s not working as i expect.
Any body help me.
Thanks