I'm thinking of scaring my young cousin with a Screamer video
Is there a reason that you want to show her the videos?
I want to show her how scary they are.
That is not very nice. You shouldn't show her those videos as it is not kind to scare children like that.
Relax, it's preteens, not real girls
Preteens is still a little young for you to show those videos. You should wait until they are older because it is not nice to scare children like that.