I don’t know who needs to hear this, but:
Anytime someone who is not a blood relative tells your child, “You can’t tell your parents about this” that should be an immediate red flag.
Your child should immediately be removed from that person’s influence, and that person should be driven from their position of power.
Yes, I am talking about teachers. And doctors. And priests. And nurses. And politicians. And police.
There can be an innocent reason for a blood relative to want parents not to know about a forbidden movie, or a dessert, or a range trip, or a thrill ride, or any number of other things.
There can also be terrible reasons for blood relatives to want parents not to know, but those aren’t the point of this post.
THERE IS NEVER A GOOD, VALID REASON FOR A CONTRACTED EMPLOYEE OR PUBLIC SERVANT TO WANT YOUR CHILDREN NOT TO TELL YOU SOMETHING.
NEVER.
People who request that of your children are a threat, and your children are not safe in their presence.
Period. Full stop. End of story.

Nothing to add from this side.