I would approach professionally. Explain you didn't mind working the overtime, however you would expect to be pay during these overtime hours. because, its only fair to you and if they refuse, kindly stop work over time.
The Babysitter and the parents need to be on the same level of authority, Timeout works, taking favorite toys for a short amount of time, taking away TV, whatever seem to be most important. BUT make sure the little one KNOWS why he or she is in time out or... more
The chance to come back and watch their children. ;o}
First of all, it's important to know how the parents handle bad behavior, and definitely, follow through with the parents discipline philosophy.