Mallard Fillmore Suzanne Fields Arianna Huffington Tony Snow Michael Barone Michael Medved Ch. I believe that this is too much and probably represents an instance of child abuse.
Children are amazingly adaptive in putting up with adults' wacky behavior.
But this exceeds appropriate boundaries. From a medical standpoint, this activity has a dangerous potential. An occasional scary swing, performed cautiously, is probably not harmful. However, I worry that the activity you describe is so clearly inadvisable that your grandson may inadvertently hurt the child. He should investigate other, less dangerous recreational activities.