So I know Heroku's free dynos 'wind down' when there isn't any traffic to them– how would this effect the cron jobs that I've implemented using the node-cron module?
Jobs scheduled by node_cron won't run when your free dynos are sleeping.
As an alternative, you can use the Heroku Scheduler add-on to schedule your cron jobs. That will trigger one-off dynos to run your cron jobs. Provided you don't exceed your monthly allowance of free dyno hours, you will be able to run your cron jobs for free.