Display the children of the current page
The following snippet will give you all children of the current page and will include the title and thumbnail.
Get Post Content from a Specific Page
This will pull in the content from a specific page. Where page_id=38 is where you define the page id. Change "38" to your desired page id.
Show the Children of a Parent Page
The following snippet will output the children of a parent page. Change '000' to the page ID.