It is very common for dogs to feel as though their place has been taken. Every time the Grandparent and Grandchild are sitting or interacting together, try to make it a pleasant experience for the dog. Have Grandpa or Grandma and the Grandson or Granddaughter play with the dog or give him/her a treat while they are together. This will make the dog feel as though he/she is still special, and having a kid around isn't so bad after all. Remember not to over do it with milk bones though, maybe switch between the milk bones and carrots (if the dog will eat carrots). This must happen every time that Grandparent and Grandchild are together. The repetitiveness of the act is what will make the dog understand that this is a good thing.