YANBU - it should not be expected that grandparents will provide free childcare. When I had DS, both sets of grandparents were 10 years off retirement so I had to organise childcare and I didn't expect anything from them.
I'm now a single parent and initially XP had EOW and 2 afternoons a week. When he buggered off I was naturally going to increase DS afterschool care but my DM offered to have him on the afternoons XP would have had him after school.
DM has DS today as he is unwell so that I could still go to work, but it was an offer - I didn't expect it from her and I pack DS round there with his favourite foods etc. Anything to make it easier.
My DM is only mud sixties and enjoying her retirement now - several holidays a year and lots of socialising. If she (or DF, but he's actually still working pt) can't have DS for whatever reason (holiday, illness etc) and they offer to help with the cost of extra childcare I practically bite their hand off!
Your DD is being a bit ungrateful really - you don't have to offer anything. Grandparent is a fun role that you've earned through raising your children. You should not be an unpaid skivvy for your kids. You are still helping to raise them without being regular childcare.
Unfortunately I think there is a bit of a culture now - I know of a lot if young people who expect their parents to be unpaid childcare and the parents just do it. It means that to others looking in that that's the way it should be and they don't see it gorgeous what it is - a favour. Of course some people love it - I have a colleague who regularly looks after her DGD overnight for her DD, but she is a young healthy woman in her late 40s, another woman in our office is the same age and pregnant! But it's her choice to have her DGD and I never get any impression she's anything but happy to do it.
That's the difference - your level of involvement is a choice. Just as a woman can choose to be a SAHP or a WOHP you can choose to be as involved and hands on as you like.
Like with me and my DM, circumstances might change over time, but you shouldn't commit yourself to something before the baby has even arrived. And don't set a precedent because if you look after this one how long before she has a second, a third? Would you then be expected to look after all of them?
Stick with your choice. You are a person with a life too and you are at a time where you should be enjoying yourself (not to say you won't enjoy grandkids, but childcare is hard work and you yourself said you've got an active social life now).