Will I be sinful if I turn on the television for my parents to an Islamic program, and then they change the channel to music and movies? And do I have to turn it off before I leave if someone comes and changes the channel after I have turned it on for them?

There is no harm in tuning the television to a permissible channel. The sin of the father changing the channel to a forbidden one falls solely upon him. The father should be advised gently and kindly if he changes the channel to something forbidden. If he complies, then that is good and excellent. Otherwise, you have fulfilled your duty, and he should not be forcefully prevented from changing it.