Threads such as this always surprise me a little, because they seem to focus so much on either/or attitudes.
I am very lucky to be involved in the lives of my DC & DGC, My GS first stayed with me alone, overnight, aged 8 months. Parents were attending a wedding. It suited everyone, and I loved having him to stay BUT would have equally enjoyed having him here with DD, as its a treat to spend time with her too. Why am I lucky? well I wouldn't have let fleas stay with my mother, so having DGCs whose parents trust me is wonderful, and surely trust is where the issues lie, in all of these discussions.
I remember being a new mum, and feeling no one could care for my child as I could. I respected that in my daughter, and never once mentioned wanting care out of her sight. As a result, we spent lots of time with her around while I played with DGC and she watched how I handled comforting her little one/treated her views on foods/etc She developed total trust in my care of her child.
On the other hand, her IL's would visit but act as "visitors" and not really engage with DGC in her presence, always standing back. If he cried, they handed him straight to her. They are lovely people, and adore their grandchild, but did absolutely nothing to gain their DILs trust in their ability to comfort & care for her child. Yet they pestered incessantly to have him "to themselves".The more they pressed the more "anti" she became because she felt pressured.
Some years down the line, he does now stay with them but it took far longer than it should have done, simply because they did nothing to establish faith in their child caring abilities, while pushing for sole responsibility.
Isn't that enough to give any young mother the heebie-jeebies.
Having DGC to stay, alone, is certainly a bonding time .... but bonding with them in front of their mother is the biggest step a grandparent can take to ensure that the parents do actually entrust their children to grandparents care.
Why is it always MILs that seem to fall foul of this? well (all other parameters being normal) they raised their son but the mother of their DGC has absolutely no idea how they parent until she sees it in action.