You say the daughter with the dog is your eldest, but you don't mention whether she is the mother of any of the grandchildren you have been looking after. If she is not, she probably feels reasonable about asking you to take her dog on, when you have looked after her nieces and nephews.
These days lots of childless people equate dogs with children, so if this is the case she will be hurt or offended when you explain that you are not able to help with her dog.
I agree with all the others: looking after children is one thing, looking after a dog is something different. I don't suppose your daughter asked whether you would be willing to help with the dog before she got it, did she?
It should be possible to leave a dog alone while at work if it has been walked in the morning, especially if someone comes in to walk the dog in the middle of the day. SOMEONE PAID TO DO SO - NOT YOU
Tell her as kindly as possible that you are looking forward to being free of family responsibilities and that you want to be able to do a lot of exciting things while you are still able, now that you have retired and are no longer the family's babysitter cum Nanny.
If she is hurt or offended that's her problem, please don't let her make it yours. Probably both of you feel the other is being unreasonable here, but you can't tell me you got through the teenage years without this young woman and her siblings feeling you were unreasonable and telling you so, and you all got over that, didn't you? Feel free to point that out to her!