get_children() returns ALL attachments instead only those attached to ‘post_parent’

hat I do: in my template that I use to display posts within the standard loop, I make a call to above function: fs_gallery_featured_thumbs( $post->ID ) (originally I used the_ID() instead of $post->ID which gave me trouble. I guess bescause the_ID() actually echoes the ID, rihgt?).

Exactly. The problem isn’t in your function itself, but rather in the parameter you pass to it. Because the_ID echoes rather than returns its value, the actual function was getting passed NULL, resulting in attachments for all posts being queried.

Solution:

• Pass $post->ID (in a context where $post is globalized)
• Pass get_the_ID() (inside the Loop)