Is there a simple way to synch up two AWS RDS Mysql Instances?
Check out AWS Database Migration Service (DMS) - that's what you need. It can run continuously, replicating Prod to Dev. Or you can run it manually from time to time, but be aware that if you use the Dev DB the subsequent sync may fail due to conflicts.
Another possible option is to create a Read Replica for your Prod database, let it sync, and then disconnect and promote to be the Dev DB.
Hope that helps :)