Everyone's circumstances are different and reading Izabella's post, if money is tight, then I think it's quite in order to point that out to the child's parents, after all grandparents' childcare is free, and no doubt, top quality
We have just started looking after our grandchldren for a short while a couple of afternoons a week to fit in with their mother's working schedule, sometimes this involves picking the older one up from school and then giving them both their tea. Quite happy to do that we are near about five miles away and don't need to be reimbursed for petrol, but would not judge others for asking for a contribution. We've had to help our children financially from time to time, but we are fortunate we both inherited money from our own parents, some of which we have earmarked to help our kids and grandchildren, so long as it's used sensibly. It's getting the balance right really, everyone has their own idea of what is reasonable and how much of an input they are prepared to give without it impacting on their own life or finances. I have read posts on GN before where some sound really put upon and I sympathise with them. My perception is that this generation of grandparents do far more than previous generations, some happily and some under sufferance. The upside of a lot of involvement I guess is that it will bode well for a closer relationship.