Family relationships are always complex, and the relationship between adult children and their parents generally do not become any less complex by nature of the aging of the children into adulthood. Sometimes, it merely makes them more complex, as the children may grow to resent the "suggestions" of parents, finding them intrusive and meddlesome.
And while grandchildren can add even more potential for irritation and conflict, the mix can become very roiled by the occurrence of divorce. As a grandparent, you may be faced with many additional complexities, as you attempt to maintain viable relations with all parties.
The difficulty for the grandparent is avoiding alienating either parent of your grandchildren. Because their decisions regarding their child custody arrangement will control how and when you have access to your grandchildren, maintaining positive relationship with both parents is important.