This question has come up with increasing frequency. The answer is actually very little under the prevailing California family code which is 3100-3105 The court has the power to grant vistation to a grandparent IF a grandparent petitions the court in the appropriate manner. Biological parents always have a preferential right under that code and no order may be made that interferes or conflicts with the rights of the biological parent to their child.
Normally there should not be an issue in involving any grandparent in a child’s life, without a court order, as reasonable parents can put their childen first. The issue becomes relevant when the grandparent is involved in abuse, covers up for the psychopathic behavior of a parent, takes a child without the consent of the other parent to an undisclosed location and sabotages and detrimentally interferes with the relationship that the children have with their biological parent and destroys any contact that the children have with other extended family members as THAT grandparent is the only one who counts, regardless of the cost to the children or the biological parent involved
There is a case in San Bernardino county where the judge issued an order prohibiting all contact with the paternal grandparents citing that the grandparents are toxic to the children in the case. The court did not have authority to make the order as the grandparent didn’t petition the court for visitation pursuant to CA family code 3100-3105, the grandparents are not parties to the case and both parents do not oppose contact with the grandparents. That did not stop the court from issuing the order.