Sorry, but parents of the child have priority over the grandparents.  However, smart schools set aside an area for parents of the performing class to stand/squat/sit and video/take photos, ususally just to the sides of the standard seated areas.  Heck, if only the parents were allowed to attend May Day, there would be no crowding issues at all.  Maybe that should be the solution.  Kupuna can watch the video afterward.