I added the child theme, but Roboto Slab is not rendering? got any tips for me?

<?php

if(isset($avia_config['use_child_theme_functions_only'])) return;

add_filter( 'avf_google_heading_font', 'avia_add_heading_font');
function avia_add_heading_font($fonts)
{
$fonts['Roboto Slab'] = 'Roboto+Slab:100,300,400,700';
return $fonts;
}

add_filter( 'avf_google_content_font', 'avia_add_content_font');
function avia_add_content_font($fonts)
{

$fonts['Roboto Slab'] = 'Roboto+Slab:100,300,400,700';
return $fonts;
}