Disable URL rewriting logic for HTTPS detection in WordPress 5.7 on your child site.
Snippet Type
Execute on Child Sites
Snippet
remove_filter( 'the_content', 'wp_replace_insecure_home_url' );
remove_filter( 'the_excerpt', 'wp_replace_insecure_home_url' );
remove_filter( 'widget_text_content', 'wp_replace_insecure_home_url' );
remove_filter( 'wp_get_custom_css', 'wp_replace_insecure_home_url' );