1. Copy header.php and content.php from the parent theme directory to your childtheme directory
2. Open header.php (childtheme) and find
<hgroup>
<h1 id="site-title"><span><a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></span></h1>
<h2 id="site-description"><?php bloginfo( 'description' ); ?></h2>
</hgroup>
3. Replace with
<hgroup>
<?php if (is_front_page()) { ?>
<h1 id="site-title"><span><a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></span></h1>
<h2 id="site-description"><?php bloginfo( 'description' ); ?></h2>
<?php } else { ?>
<h2 id="site-title"><span><a href="<?php echo esc_url( home_url( '/' ) ); ?>" title="<?php echo esc_attr( get_bloginfo( 'name', 'display' ) ); ?>" rel="home"><?php bloginfo( 'name' ); ?></a></span></h2>
<h3 id="site-description"><?php bloginfo( 'description' ); ?></h3>
<?php } ?>
</hgroup>
4. Open content.php (childtheme) and find
<h1 class="entry-title"><a href="<?php the_permalink(); ?>" title="<?php printf( esc_attr__( 'Permalink to %s', 'twentyeleven' ), the_title_attribute( 'echo=0' ) ); ?>" rel="bookmark"><?php the_title(); ?></a></h1>
5. replace with
<h2 class="entry-title"><a href="<?php the_permalink(); ?>" title="<?php printf( esc_attr__( 'Permalink to %s', 'twentyeleven' ), the_title_attribute( 'echo=0' ) ); ?>" rel="bookmark"><?php the_title(); ?></a></h2>
Does that work for you?