I'm sorry you find yourself in this painful position. As hard as this situation is for you, parents ultimately decide who spends time with their children. The best thing is to work out things with the parents. If you can't, then you need to respect their wishes for their minor children. Don't try to bypass the parents to get to their children.
Not knowing the details of the situation it's hard to be sure whether you can have distant contact like sending cards or gifts but, if the parents aren't passing them on, their wishes there are clear.
You can go for visitation through courts if you had a previous involved relationship with the gc, but this move, successful or not, will probably be the end of any hope of a relationship with the children's parents. The stress of this may also have a negative impact on your gc.