I would report my daughter and son-in-law to child services if they carried on with this sort of behaviour. That is disgusting purposely frightening a child. It is a control thing they are trying to exert over a defenseless baby. We are supposed to nurture and care for children, not abuse them. They should not be allowed children! That is NOT a game, it is abuse!
Dear Lola,
This activity is certainly ill advised. Any behavior that deliberately startles a child has nothing to recommend it, even if the child laughs. This sort of interaction breeds anxiety and unsettledness in a youngster; it is really incumbent on parents who love their child to stop this right away.