> My 10 year old hates scary things, and she gets completely freaked out when they show scary movie previews. I would like to be able to block those for her.

The difference between this and the usual "parental control" mechanisms is that what you're describing here is something the child wants to cooperate with, voluntarily. In which case, you don't need a mechanism that makes it absolutely impossible; you need a mechanism for helping them not see things they don't want to see. That's something some adults also want (e.g. tools for preventing oneself going to Facebook, or going to TVTropes for too long).