Displaying Child pages of the current page in post format and their thumbnails (featured image)?

You’re not inside a regular loop, or at least not one where global post variables are set, so the_post_thumbnail does not have an ID to fetch a thumbnail for..

Use get_the_post_thumbnai( $page->ID ) instead and it should work fine.

Hata!: SQLSTATE[HY000] [1045] Access denied for user 'divattrend_liink'@'localhost' (using password: YES)