In your child function.php file:
function abyline_posted_on() {
$link= esc_url( get_permalink() );
$title= esc_attr( get_the_title() );
$time= esc_attr( get_the_time() );
$dateGMT= esc_attr( get_the_date( 'c' ) );
$date= esc_html( get_the_date() );
$authorURL= esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) );
$author= esc_html( get_the_author() );
echo ('<span class="sep">Posted on</span> <a href="'.$link.'" title="'.$title.'" rel="bookmark"><time class="entry-date" datetime="%3$s" pubdate>'.$date.'</time></a> <span class="sep"> by </span> <span class="author vcard"><a class="url fn n" href="'.$authorURL.'" title="'.$author.'" rel="author">'.$author.'</a></span>');
}
*You can name the function anything you want as long it matches the call in the content.php file. I call my function 'abyline_posted_on'
-----------------------
In your child content.php
Look for this section:
<header class="entry-header">
<?php the_post_thumbnail(); ?>
<?php if ( is_single() ) : ?>
<h1 class="entry-title"><?php the_title(); ?></h1>
******* <?php abyline_posted_on(); ?><!-- this is the first call to the byline function for single posts -->
<?php else : ?>
<h1 class="entry-title">
<a href="<?php the_permalink(); ?>" title="<?php echo esc_attr( sprintf( __( 'Permalink to %s', 'twentytwelve' ), the_title_attribute( 'echo=0' ) ) ); ?>" rel="bookmark"><?php the_title(); ?></a>
</h1>
****** <?php abyline_posted_on(); ?><!-- this is the 2nd call to the byline function -->
<?php endif; // is_single() ?>
*Note* I wrap my byline calls in a DIV to style the text but that is up to you