Thumbnail size in child theme

Hi, guys! Thank you for your perfect work. I love Nepal. I was in track several times (around Annapurna) and now I am happy to have your theme on my website. Can you advise me how to do proper change of thumbnail size in the excerpts in child theme? I need them to be square 150x150px. Is it the only way to copy parent functions.php to child theme and to rewrite whole catchbox_setup function there? Is there an easy way?