Child Theme

I have been using a plugin to create a child theme of Crimson Rose and this error message pops up:

Fatal error: Cannot redeclare crimson_rose_css_set_unit() (previously declared in /home/beemboutique/public_html/wp-content/themes/crimson-rose/css/mixins.php:23) in /home/beemboutique/public_html/wp-content/themes/crimson-rose/css/mixins.php on line 23

Any support would be greatly appreciated!

Thanks