Would the questioner be sinful for preventing his parents from watching television—under the pretext of his children's corruption—thereby causing them distress?

Preventing the television from being turned on is not considered disobedience (to parents). However, if the parents feel bored and wish to turn it on, there is no objection, provided forbidden things are avoided, as there are beneficial channels free from violations. Alternatives can be resorted to, such as computers, to display beneficial programs and artistic works that are far from forbidden things.