VioletSky
It's a safeguarding concern
You pass it on straight away.
You don't reply to it and give grandson the chance to delete it
You don't have a conversation with grandson about it before the parents giving the opportunity for them to beg
You don't run to an Internet forum for it to be discussed
You don't send a message to son that contains your personal description of something he can't see.
What you do is forward the video with a "grandson sent this by accident and I thought you should know
Because otherwise you just started a big drama in an already problematic relationship with someone who already doesn't trust your word
Well yes I agree with uou VS
However by the time the OP posted she had already had a discussion with grandson and, it appears, deleted the video. She can't go back and redo her initial response, therefore going forward the answer to her question "Do I tell, or do I keep quiet" is "Tell"