I have looked after my DGC for countless hours, and because I was not well off, my DD insisted on paying me.
It made her feel happier, and I had money to spend on little outings, treats, and clothes for the little ones.
I am better off now, so flatly refuse any money. My daughter has accepted that so long as I have the means, having the children’s company is payment enough. As far as I’m concerned, any spare cash I have is for my DGC.
This is something that has to be worked out on an individual basis. If you are badly off, a little extra money will ensure better and more enjoyable times with your grandchildren, if you are reasonably flush, then accept it if it makes DIL happy, and save it up in a separate account for the kiddies.