I'm not sure why you don't have that file. I found it on a fresh installation through https://aws.bitnami.com/. Maybe you used a different approach to launch your image.
Anyway, my suggestion is to run this:
egrep -Ri pm.max_children | grep 5 | grep -v 15
This will return the list of files that contain this parameter with value 5. I think it should be enough with editing
etc/php-fpm.d/www.conf.default, but you can edit all of them (in case you have more than one) to use your desired value. After that, execute:
sudo /opt/bitnami/ctlscript.sh restart php-fpm
I hope it helps