That is very unfortunate. Although you don’t want to put them in a nursing home, or separate them permanently, what you could do is send one of them to adult day care, or schedule outings for them separately so that they have some time apart. This could help relieve some tension between them. Also, try talking to your dad about his language and the example he’s setting for his grandchildren. Although many elderly are set in their ways, it could soften him to think of it from a child’s perspective.