Targeting child pages of custom post types using function?

is_singular only accepts one argument- post type. It’s only for checking if a post is of a certain type(s), not a specific post of that type. Check $post->ID instead-

if(is_singular('guides' )&&($post->post_parent==$pid||$post->ID==$pid))

I would also consider using get_queried_object() and/or get_queried_object_id() rather than use the global $post.

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