I have 5 main posts within a custom post type. Some of these posts will have children. I have content that should only be displayed on the parent post and all children of that post. What's the best way to achieve this?
You could setup a condition based on post ID:
If you don't know how to locate the post ID:
"Roll over any page, post or category with your cursor (while logged in and in the "manage" area of course), and the page, post or cat_id number appears in the progress bar in the bottom of the browser." - http://wordpress.org/support/topic/where-is-the-post-id-number
Your question is a little unclear...I think what you're trying to ask is how to display the parent content for that post...and then also the content for each child?
FOR THE PARENT (let's say your content is the title)
FOR THE CHILDREN
The whole thing together will look like this: