Hi MiguelG,
PHP_FCGI_CHILDREN can not be put into wrapper, simply because it is a binary file:
/var/www/cgi-bin/cgi_wrapper/cgi_wrapper
However, you can try adjust server-wide setting (FcgidMaxProcesses, will be applied to all sites)
/etc/httpd/conf.d/fcgid.conf
FcgidMaxProcesses 20