I'm reiterating a couple of the comments above, I too wonder if the child care is given freely, or whether the grandparents were presented with a fait accompli when baby was born and there was an assumption that a grandparent/s would automatically give their time, if so, there could be a feeling of being taken for granted and a simmering resentment.
I'm amazed at how much grandparents do, I'm not sure previous generations paid such a pivotal role in child rearing, my grandparents certainly didn't and my mother went back to work full time when I was half way through junior school, but then attitudes were more lax then I think. We just came back from school and fended for ourselves until my mother returned. Of course that wouldn't be possible for very young children, and I'm not sure I'd have been happy with my children of say 8 and 10 being alone in the house for a couple of hours after school, but it wasn't unusual then. Can't even remember what our parents did in the holidays, think they must have shared it a bit and do remember spending a lot of time at a friend's house.
My husband has brought up 2 families, 4 children in total, and then stepped in and played an important role in his granddaughters lives when his son died and is unequivocal at this time of his life, now he is retired, he wants us to be free to go away at the drop of a hat and refuses to be tied down to "set in stone" child minding. We do a once a weekish sleepover with gd aged 5 and have stepped in for emergencies looking after her and baby brother when it was needed. We have also given them a lot of help financially. I'm wondering whether grandparents who take over a large part of the child rearing were ever consulted in the "planning" stages as to whether this is something they want and are able to commit to.
If arrangements have been put in place for child minding between two sets of grandparents then for that to work it would seem essential that they liaise with each other and give due notice for breaks etc. Nevertheless, ultimately it should be down to the parents to look after their own children if their parents, for any reason can't, In particular, I'm agreeing with MamaCaz's comments regarding holidays.