Hi, and welcome to this forum.
I would respectfully ask my parents to show you first the video to see if you approve of your child viewing it. A lot of the videos are very good lessons about how to get along with others and behavioral stuff.
Also be prepared to counter the indoctrination with reasoning a child can understand, like how fun certain celebrations are, etc.
If parents do not heed your words, there is always strictly supervised or minimum visitations.
DY