Childe Menu and posts from one category

Hi, I’m trying to set the child menu site withe the last posts form only one category. How to do this? That they show properly like on the home page? The easy way is to add category to the menu bar – but how to do this for child menu on the sidebar?

I paste a code

[posts-for-page cat_slug='op-razem' num='5' read_more='<br>Czytaj wi?cej &raquo;' show_full_posts='false' force_excerpt_image='true' use_wp_excerpt='true' hide_post_content='false' show_meta='true' hide_post_title='false' order_by='date']

to get the posts but it works only in a half way – the posts are sown not so nice as on the home page…