As others have said you need a full and frank discussion as to what you both want from retirement and how to apportion your time.
My husband is hands on when our grandchildren come to us, although he does stress there are limitations as to how often he wants to do child care, certainly not four days a week. He's had 4 children and has 5 grandchildren, although only 2 of each are ours together, undoubtedly family is a very important part of his life, but so is his hobby, golf, which he plays 3 times a week to keep fit, plus the male camaraderie sid of things. I think life is about balance ideally. Time for yourself, for each other and for family and friends, anything can become all encompassing whether it be a sport or looking after grandchildren. Grandparents have been through it all once, to have it again to almost the same degree, could be a bone of contention, not everyone wants to be a "professional grandparent".
Sulking is difficult to deal with though, especially from the grandparent 