Grandparents generally do not have the same visitation rights as a non-custodial natural parent.   A typical visitation schedule for a divorced father, for example, might state:  "Every Other Weekend, Alternate Holidays, Extended Summer Visitation", etc.   Grandparents are pretty much at the mercy of the parents of the grandchildren when it comes to visitation.  

You can get more details at my blog on the subject at this website.