most states don't grant g-parents
visitation rights unless their child is deceased (the child's parent who is the child of the g-parents) or the parent is incarcerated. in Minnesota, if they live with you for more than a year and move out of your house, you can petition for visitation. here's a link I found:
http://www.uslegalforms.com/lawdigest/grandparents.php/MN/MN-DO-30-FF.htmif I were you, and the child is not in danger, I would not take sides. Your son and gfriend are very young. with your encouragement, as your grandchild's advocate, I would try to get my kids to see the light and let the g-father see his g-child once in a while. if the kids want nothing to do with him, maybe you could be the 3rd party to facilitate this, with their permission, taking the g-child over there to see g-pa.
put the shoe on the other foot, how would you feel if they got pissy with you,
moved out of your house, and decided that you couldn't see your grandchild anymore? you wouldn't like that, would you? without any more info or details from you, these kids sound spoiled and immature. they're very good at biting the hand that feeds them.