Confusedgranny, I do understand where you are coming from. Of course we all want to be a part of our grandchildren's lives, but it's not for us to decide when or how often we see them, especially when our sons/daughters lead busy lives, and spend most of the week at work, with little time to enjoy relaxing together with their own little family.
Some grandparents get to see their GC far less than you do ... and some see them far more than they would like!Lol You suggest picking up your GC on a Friday and cooking dinner for your DS & DiL ... but don't you think that, after working all week, they probably relish the thought of spending Friday night relaxing alone, perhaps cooking a meal together, or ordering a takeaway, and enjoying a bottle of wine after the children have gone to sleep ...? I know that's what my DS and DiL enjoy doing, and I certainly wouldn't want to intrude on that time by expecting to stay with them for the weekend. In my view, that wouldn't be helping them ... it would be encroaching on their valuable time that they like to spend together at weekends. My son & DiL live overseas, so I don't get to spend a lot of time with my GC, although we do skype regularly, and I visit occasionally for a few days, but only when it's convenient for them.
Many of us find it difficult to accept that when our sons/daughters leave home, and have their own families, their parents are no longer the most important people in their lives, and we need to let go and let them get on with their own lives without interfering, or expecting too much from them. As others have said, try taking a step back. Tell them you look forward to visiting again, but ask them to let you know whenever it would be convenient, as you appreciate they are busy and need their own time together.